Remove Javadoc not failing on error (#7258)

The 'javadoc' task should fail when there is an error. The
'javadocCleanup' task failed for PR 7239 due to a plugin issue 242,
while it should have been the 'javadoc' task that failed the build.
Also, publishing when javadocs fail is not desired.

Author: benjamintboyle

build.gradle

@@ -66,7 +66,6 @@ tasks.withType(JavaCompile) {
 apply from: file("gradle/javadoc_cleanup.gradle")
 
 javadoc {
-    failOnError = false
     exclude "**/internal/**"
     exclude "**/test/**"
     exclude "**/perf/**"