Failsafe version of checkVersion Task

Prevent stopping  check Task if an exception happens during checkVersion execution.
Print proper warning message if a version violates semantic versioning.

Closes asciidoctor#479

gradle/versioncheck.gradle

@@ -18,35 +18,75 @@ task checkVersion() {
         throw new GradleException("I need the gem name to check the version. Add a 'gem_name' property to the gradle.properties file. (e.g.: gem_name=asciidoctor-diagram)")
     }
 
-    def gem_version_pattern = ~/(((\d+\.){1,2}\d+)(\.)?(.*))/
-    def gem_latest_version_url = "https://rubygems.org/api/v1/versions/${gem_name}/latest.json".toURL()
+    doLast {
 
-    def json = new JsonSlurper().parse( gem_latest_version_url )
-    def gem_latest_version = json.version
+        String gem_latest_version = getLatestGemVersion(gem_name)
+        String javaVersion = latestGemVersionAsJavaVersion(gem_latest_version)
 
-    def matcher = gem_latest_version =~ gem_version_pattern
-    def javaVersion = gem_latest_version
+        try {
 
-    doLast {
+            Version projectVersion = Version.valueOf(version as String)
+            Version gemJavaVersion = Version.valueOf(javaVersion)
 
-        if ( gem_latest_version ==~ gem_version_pattern ) {
-            matcher.find()
-            if (matcher.group(4)) {
-                javaVersion = "${matcher.group(2)}-${matcher.group(5)}"
+            if (projectVersion.lessThan(gemJavaVersion)) {
+                logger.warn "\nWARNING\nVersion mismatch: Current Version: {}, Gem Version: {}\n",version , gem_latest_version
+            } else {
+                println "up-to-date"
             }
         }
+        catch (Exception e) {
+            logger.error "\nWARNING\nSemantic version violation: Current Version: {}, Gem Version: {}\n",version , gem_latest_version
+        }
+    }
+}
+
+/**
+ * Consume rubygems webservice to determine latest gem version
+ *
+ * @param gemName The gem's package name. eg.: asciidoctor-diagram
+ * @return latest gem version as String
+ */
+private String getLatestGemVersion(String gemName) {
+    def gemVersion
 
+    try {
+        def gem_latest_version_url = "https://rubygems.org/api/v1/versions/${gemName}/latest.json".toURL()
+        def json = new JsonSlurper().parse(gem_latest_version_url)
+        gemVersion = json.version
+    }
+    catch (Exception e) {
+        logger.error "Problems getting latest gem version. {}", e.message
+        throw new StopExecutionException()
+    }
+    gemVersion
+}
 
-        Version projectVersion = Version.valueOf(version)
-        Version gemJavaVersion = Version.valueOf(javaVersion)
+/**
+ * Transforms a gem version into a java version representation.
+ *
+ * e.g.: The gem version of asciidoctorj-epub3 1.5.0.alpha.6 transforms to 1.5.0-alpha.6
+ *
+ * @param gem_latest_version The latest gem version as String
+ * @return a java compatible presentation of a gem version
+ */
+private String latestGemVersionAsJavaVersion(String gem_latest_version) {
+    def gem_version_pattern = ~/(((\d+\.){1,2}\d+)(\.)?(.*))/
+    def matcher = gem_latest_version =~ gem_version_pattern
+    def javaVersion = gem_latest_version
 
-        if ( projectVersion.lessThan(gemJavaVersion) ) {
-            logger.warn "\nWARNING\nVersion mismatch: Current Version: $version, Gem Version: $gem_latest_version\n"
-        }
-        else {
-            println "up-to-date"
+    try {
+        if (gem_latest_version ==~ gem_version_pattern) {
+            matcher.find()
+            if (matcher.group(4)) {
+                javaVersion = "${matcher.group(2)}-${matcher.group(5)}"
+            }
         }
     }
+    catch (Exception e) {
+        logger.error "Problems transforming gem version {} to java version", gem_latest_version
+        throw new StopExecutionException()
+    }
+    javaVersion
 }
 
 checkVersion.onlyIf { !gradle.startParameter.isOffline() }