Handle hgrid exception raised when dropbox token is invalid

An unhandled error occurs if a user tries to drag and
drop a file into a linked dropbox folder when their
credentials are invalid.

Re-raise the dropbox.rest.ErrorResponse [401] as
an HTTPError. If the error is not 401, raise as
400.
Test upload dropbox file with invalid token throws error

Also change error message language to include a
more general "check your project settings"
to apply to both the dropbox account owner
and any contributors.
Merge branch 'hotfix/0.21.8'