[FIX][DOC] Provide more explanatory error message. TODO: when uploading in tracker edit dialog, entire dialog closes. Ideally, there should be a way to show the error without aborting the whole edit dialog.

This fix is for master, 19.x and 18.x.

If you turn the "Upload files from remote source" feature on, then if you setup a tracker with a Files field and then try create a tracker item, the Create Item modal dialog appears:

If you enter an invalid URL in the "Upload from URL" field and then press enter, leading to a failure to get a file, a Services Exception is thrown, which causes the dialog to close. There is an error message shown on the page "Data could not be obtained". I've changed the message to something better.

Ideally it would be best if an error could be shown in the Upload File dialog instead of closing the dialog and showing the error on the underlying page (because if the user is editing the tracker with many fields he would lose whatever he was editing), but I can't figure out how to handle Services_Exceptions that way, so I've simply changed the error message for now. If you know how to better handle Services_Exceptions from dialogs, please let me know.