BrowserActivity: handle case where no path is returned for file to upload

When the user chooses the "Other" upload method, and then selects an image,
the path will be null, and it will cause a nullpointer in BrowserActivity.
Rudimentary avoid this NPE by showing a message to the user, and log it.
Merge pull request #87 from alfh/bug_exception_upload_image

BrowserActivity: handle case where no path is returned for file to upload