Improve test so that snippet is displayed when an exception is thrown

Change-Id: Ib5a7ed985df42b7d56dd98e104a145fddc895599
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/105020
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/test/verify_docs_test.dart

@@ -136,6 +136,14 @@ $snippet
           writeError(error, importsLength);
         }
       }
+    } catch (exception, stackTrace) {
+      if (output.isNotEmpty) {
+        output.writeln();
+      }
+      output.writeln('Exception while analyzing "$snippet"');
+      output.writeln();
+      output.writeln(exception);
+      output.writeln(stackTrace);
     } finally {
       provider.removeOverlay(snippetPath);
     }