This user script (aka greasemonkey script) adds a "Print Error" button to the upper right corner of Marvel Sense. (It's a ! with a circle around it.)
When clicked, it looks in the output pane and checks if the body of the output looks like an ES error. If it does, it'll display it in a slightly more readable form (adds pretty line breaks, etc.).
Subsequent clicks of the Print Error button will update the error message if the content in the output has changed or hide the error message if the content in the output doesn't look like an error.
Use Greasemonkey in Firefox or Tamplermonkey in Chrome to install it.