Catch basic configuration errors and display to user.

E.g. If the asset builder has no "Asset Types to Create" previously this
plugin would appear to be uploading the files, but nothing would
actually get created in Matrix. Now at least the text from Matrix is
appended to the screen:

   The new asset was not created. Ensure you have entered all required
   information and try again.
    * Create type is not allowed

This could probaly do with a cleaner strategy, but as a first pass it
works.
Ensure upload size does not exceed 100%

With small files the percentage displayed could exceed 100% (as the code
is counting bytes used transmitting headers), this just caps the display
value to avoid confusing the user.
Merge pull request #1 from neerolyte/catch_matrix_rejections

Catch basic configuration errors and display to user.
Merge pull request #2 from neerolyte/upload_exceeds_100_percent

Ensure upload size does not exceed 100%