Don’t display tasks if the build file can’t be compiled.

src/main/kotlin/com/beust/kobalt/Options.kt

@@ -108,7 +108,9 @@ class Options @Inject constructor(
                 }),
                 Option( { args.tasks }, {
                     // --tasks
+                    runIfSuccessfulBuild(buildError) {
                         displayTasks()
+                    }
                 }),
                 Option( { args.checkVersions }, {
                     // --checkVersions