Fix DefaultBackportService.findMilestoneTitle for errors

The `this.github.findFile()` produces a `Mono.error` when file not found instead of `Mono.empty()`

* Fix `DefaultBackportService.findMilestoneTitle()` to use `onErrorResume()` instead of `switchIfEmpty()`
* Adjust `DefaultBackportServiceTests` mocks to emit `Mono.error(HttpClientErrorException(HttpStatus.NOT_FOUND))`
instead of `Mono.empty()` to simulate real HTTP result for `findFile` GH API

Author: artembilan

src/main/kotlin/io/spring/github/event/DefaultBackportService.kt

@@ -66,23 +66,25 @@ class DefaultBackportService(val github: GitHubApi) : BackportService {
                     p.load(file)
                     p.getProperty("version")
                 }
-            	.switchIfEmpty(Mono.defer {
-                    this.github.findFile(ref, "pom.xml")
-                    .map { file ->
-                        val builderFactory = DocumentBuilderFactory.newInstance()
-                        val builder = builderFactory.newDocumentBuilder()
-                        val xmlDocument: Document = builder.parse(file)
-
-                        val xPath: XPath = XPathFactory.newInstance().newXPath()
-                        var version = xPath.compile("/project/properties/revision").evaluate(xmlDocument)
-                        if (version == null) {
-                            version = xPath.compile("/project/version").evaluate(xmlDocument)
-                        }
-                        version
+            	.onErrorResume { _ ->
+                    Mono.defer {
+                        this.github.findFile(ref, "pom.xml")
+                            .map { file ->
+                                val builderFactory = DocumentBuilderFactory.newInstance()
+                                val builder = builderFactory.newDocumentBuilder()
+                                val xmlDocument: Document = builder.parse(file)
+
+                                val xPath: XPath = XPathFactory.newInstance().newXPath()
+                                var version = xPath.compile("/project/properties/revision").evaluate(xmlDocument)
+                                if (version == null) {
+                                    version = xPath.compile("/project/version").evaluate(xmlDocument)
+                                }
+                                version
+                            }
                     }
-                })
+                }
                 .map { it.replace(".BUILD-SNAPSHOT", "").replace("-SNAPSHOT", "") }
-                .switchIfEmpty(Mono.error { IllegalStateException("Cannot find 'gradle.properties' or 'pom.xml' for $ref") })
+                .onErrorResume { e -> Mono.error { IllegalStateException("Cannot find 'gradle.properties' or 'pom.xml' for $ref", e) }}
     }
 
     private fun findMilestoneNumberByTitle(repositoryRef: RepositoryRef, title: String): Mono<Int> {

src/test/kotlin/io/spring/github/DefaultBackportServiceTests.kt

@@ -23,6 +23,8 @@ import io.spring.github.event.IssueEvent
 import io.spring.github.event.PushEvent
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
+import org.springframework.http.HttpStatus
+import org.springframework.web.client.HttpClientErrorException
 import reactor.core.publisher.Flux
 import reactor.core.publisher.Mono
 import reactor.test.StepVerifier
@@ -101,16 +103,20 @@ class DefaultBackportServiceTests {
 
     @Test
     fun findMilestoneNumberWhenFindFileEmptyThenError() {
-        whenever(github.findFile(branchRef, "gradle.properties")).thenReturn(Mono.empty())
-        whenever(github.findFile(branchRef, "pom.xml")).thenReturn(Mono.empty())
+        whenever(github.findFile(branchRef, "gradle.properties"))
+            .thenReturn(Mono.error(HttpClientErrorException(HttpStatus.NOT_FOUND, "'gradle.properties' file not found")))
+
+        whenever(github.findFile(branchRef, "pom.xml"))
+            .thenReturn(Mono.error(HttpClientErrorException(HttpStatus.NOT_FOUND, "'pom.xml' file not found")))
 
         StepVerifier.create(backport.findMilestoneNumber(branchRef))
             .verifyErrorSatisfies {e -> assertThat(e).hasMessage("Cannot find 'gradle.properties' or 'pom.xml' for BranchRef(repository=RepositoryRef(fullName=rwinch/test), ref=1.0.x)")}
     }
 
     @Test
     fun findMavenMilestoneNumber() {
-        whenever(github.findFile(branchRef, "gradle.properties")).thenReturn(Mono.empty())
+        whenever(github.findFile(branchRef, "gradle.properties"))
+            .thenReturn(Mono.error(HttpClientErrorException(HttpStatus.NOT_FOUND, "'gradle.properties' file not found")))
         whenever(github.findFile(branchRef, "pom.xml"))
             .thenReturn(Mono.just("""
                 <?xml version="1.0" encoding="UTF-8"?>