Treat bad values passed to --images as a fatal error

Treat bad values passed to --images as a fatal error

If an option is passed to --images that is either a non-existent path or
a folder with no images matching the supported types, assume this is
an error and exit, so they can supply a valid path instead.

Share code between DM and nanobench in SkCommonFlags.

nanobench now behaves more like DM - it will check a directory for
images that match the supported extensions.

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Review URL: https://codereview.chromium.org/1611323004