app, ipython#319: re-raise alias/flag help-msg errors

+ After @minrk codereview: ipython#319 (comment)

traitlets/config/application.py

@@ -331,7 +331,9 @@ def print_alias_help(self):
                 lines.append(indent("Equivalent to: [%s]" % longname))
             except Exception as ex:
                 self.log.error('Failed collecting help-message for aias %r, due to: %s',
-                               alias, ex, exc_info=1)
+                               alias, ex)
+                raise
+
         # lines.append('')
         print(os.linesep.join(lines))
 
@@ -358,6 +360,8 @@ def print_flag_help(self):
             except Exception as ex:
                 self.log.error('Failed collecting help-message for flag %r, due to: %s',
                                flags, ex, exc_info=1)
+                raise
+
         # lines.append('')
         print(os.linesep.join(lines))