improve error message (#109)

useful because MelangeDerivedStateComputer tends to "randomly" fails
(ie. we don't know yet why ;-) ) and a "clean all" may help.
The message helps to track and debug the issue.

plugins/fr.inria.diverse.melange/src/main/java/fr/inria/diverse/melange/resource/MelangeDerivedStateComputer.xtend

@@ -102,16 +102,25 @@ class MelangeDerivedStateComputer extends JvmModelAssociator
 			try {
 				processors.forEach[p |
 					val pTask = Stopwatches.forTask(p.class.simpleName)
-					pTask.start
-					p.preProcess(root, preLinkingPhase)
-					pTask.stop
+					try {
+						pTask.start
+						p.preProcess(root, preLinkingPhase)
+					} catch (Exception e) {
+						log.error(" exception  in "+p.class.simpleName+ " while processing " + resource.URI+ " (may be temporary, try a clean all) ", e)
+					} finally {
+						pTask.stop
+					}
 				]
 
 				// Setting context for non-inferrer helper classes
 				builder.setContext(resource.resourceSet)
 				helper.setContext(resource.resourceSet)
 
 				task.stop
+
+			} catch (Exception e) {
+				log.error(" exception  while processing " + resource.URI+ " (may be temporary, try a clean all) ", e)				
+			} finally {
 				// Avoid computing all the derived state when unnecessary,
 				// ie. if we don't have Xbase code in transformations that
 				// relies on the code generated by Melange, we don't need
@@ -120,8 +129,6 @@ class MelangeDerivedStateComputer extends JvmModelAssociator
 					super.installDerivedState(resource, preLinkingPhase)
 				else
 					super.installDerivedState(resource, true)
-			} catch (Exception e) {
-				log.error("Fatal exception", e)
 			}
 		}
 	}