Throw error when DownloadMappings task fails

Author: LexManos

src/main/groovy/net/minecraftforge/forgedev/tasks/installertools/DownloadMappings.groovy

@@ -12,6 +12,7 @@ import org.gradle.api.logging.LogLevel
 import org.gradle.api.provider.Property
 import org.gradle.api.tasks.Input
 import org.gradle.api.tasks.OutputFile
+import org.gradle.process.ExecResult
 
 import javax.inject.Inject
 
@@ -30,6 +31,11 @@ abstract class DownloadMappings extends ToolExec {
         this.standardOutputLogLevel.set(LogLevel.INFO)
     }
 
+    @Override
+    protected ExecResult exec() {
+        return super.exec().rethrowFailure().assertNormalExitValue()
+    }
+
     @Override
     protected void addArguments() {
         this.args(