qa/tasks: change map_vips to raise exceptions instead of returning None

qa/tasks: change map_vips to raise exceptions instead of returning None

None of the callers of map_vips ever checks for a None return. So
instead of handling any error conditions it would always just blow
up with a semi-obscure TypeError. Convert the function to always
raise an exception (one that tries to breifly explain the condition)
when something goes wrong. I also take the opportunity to make
more clearer logging and reduce an indentation level.

Signed-off-by: John Mulligan <jmulligan@redhat.com>