Provide *useful* error handling
Provide *useful* error handling

Replaces http.NotFound with http.Error so we can provide messaging on
a per-issue basis (as well as non-404 errors)

This is primarily to ease debugging, but could be useful for automation
that needs to know precisely what's gone wrong - a malformed URL is very
different from a URL requesting a non-existent asset.

uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/newspaper-jp2-viewer,uoregon-libraries/rais-image-server,uoregon-libraries/newspaper-jp2-viewer,uoregon-libraries/rais-image-server,uoregon-libraries/newspaper-jp2-viewer