Create a new exception class: NuancierMultiExceptions
When generating_thumbnail do not stop on the first error

There may be situation where, even if we fail for one or more picture,
we still want to try to generate the thumbnails for the others.
With this commit, we will catch all exception that occurred while
generating the thumbnails and re-raise them at the end using the
NuancierMultiExceptions exception
When re-generating the cache, catch NuancierMultiExceptions exceptions as well
Store the MultiExceptions exception with a different variable name
Adjust the NuancierMultiExceptions exception class

This way it will not store the same messages multiple times and we won't
flash them several times as well
Catch IndexError as well when generating the thumbnails

IndexError is the exception raised when trying to process an interlaced
png image with python-PIL which apparently does not support these images.
Merge pull request #43 from fedora-infra/gen_cache

Keep going if one thing occurred while regenerating the cache