Handle errors with missing files

[BentleyJOakes]

• The response strings in the http server were crashing AToMPM because of the presence of certain characters. A cleanup function now removes these characters.
  • Note that the implementation may be slow and may be worth optimizing in the future.
• Added pop-up messages for trying to load an invalid toolbar or model, as well as if there was a problem accessing the file list.
• Added a test for the above issues.