Fix bad error handling in checkExternal

Sometimes the error message from http.Client is different in Go 1.15 for client timeouts.
The messages causing issues contain "dial tcp" which was caught before Client.Timeout.

This fix just moves the handler for "Client.Timeout" above "dial tcp".

Will close #148

htmltest/check-link.go

@@ -182,18 +182,6 @@ func (hT *HTMLTest) checkExternal(ref *htmldoc.Reference) {
 		<-hT.httpChannel // Bump off http concurrency limiter
 
 		if err != nil {
-			if strings.Contains(err.Error(), "dial tcp") {
-				// Remove long prefix
-				prefix := "Get " + urlStr + ": dial tcp: lookup "
-				cleanedMessage := strings.TrimPrefix(err.Error(), prefix)
-				// Add error
-				hT.issueStore.AddIssue(issues.Issue{
-					Level:     issueLevel,
-					Message:   cleanedMessage,
-					Reference: ref,
-				})
-				return
-			}
 			if strings.Contains(err.Error(), "Client.Timeout") {
 				hT.issueStore.AddIssue(issues.Issue{
 					Level:     issueLevel,
@@ -215,6 +203,20 @@ func (hT *HTMLTest) checkExternal(ref *htmldoc.Reference) {
 				}
 			}
 
+			// More generic, should be kept below more specific cases
+			if strings.Contains(err.Error(), "dial tcp") {
+				// Remove long prefix
+				prefix := "Get " + urlStr + ": dial tcp: lookup "
+				cleanedMessage := strings.TrimPrefix(err.Error(), prefix)
+				// Add error
+				hT.issueStore.AddIssue(issues.Issue{
+					Level:     issueLevel,
+					Message:   cleanedMessage,
+					Reference: ref,
+				})
+				return
+			}
+
 			// Unhandled client error, return generic error
 			hT.issueStore.AddIssue(issues.Issue{
 				Level:     issueLevel,