[mlir] Update signal failure error message

[jpienaar]

Summary

User was confused by previous wording when pass was reported as failing
due to signal in unrelated thread being caught.

mlir/lib/Pass/PassCrashRecovery.cpp

void RecoveryReproducerContext::crashHandler(void *) {
   // We can't know for certain which one was the cause, so we need to generate
   // a reproducer for all of them.
   for (RecoveryReproducerContext *context : *reproducerSet) {
     std::string description;
     context->generate(description);
 
     // Emit an error using information only available within the context.
     emitError(context->preCrashOperation->getLoc())
-        << "A failure has been detected while processing the MLIR module:"
-        << description;
+        << "A signal was caught while processing the MLIR module:"
+        << description << "; marking pass as failed";
   }
 }
 
 void RecoveryReproducerContext::registerSignalHandler() {
   // Ensure that the handler is only registered once.
   static bool registered =
       (llvm::sys::AddSignalHandler(crashHandler, nullptr), false);
   (void)registered;