web: support Image from PIL in testing

Change-Id: I8e7a70c30fe1859e7dd2fa32ddd1740bc17ebfd7
web: print the traceback from errors as well

Change-Id: I645343f65241d1ba8b0075be479432387977afb9
Merge topic 'web-testing-fixes' into master

2f81a646 web: print the traceback from errors as well
7cce0e53 web: support Image from PIL in testing