This is to make it easier to debug test failures involving
'wait-for-screen-text': the screendump image used for the OCR is now preserved
for inspection when 'wait-for-screen-text' fails.
* gnu/build/marionette.scm (marionette-screen-text): Return the screendump
image file as the second value. Adjust doc.
(wait-for-screen-text): Add the preserved screendump image file name to the
error message. Adjust doc.