PyGUI - Exception classes

# Exception classes

PyGUI defines the following exception classes for use by application code.

Cancel
  This exception can be raised to abort an operation, such as in response to the user clicking a Cancel button. It is silently caught by the main event loop.
  When writing code, you should keep the possibility of Cancel exceptions in mind, and make judicious use of try-finally to clean up after operations that could be cancelled part way through.
ApplicationError(message [, detail])
  This exception can be raised to signal an error that should be reported to the user. It is caught by the main event loop and a suitable alert displayed.
  The message should be a brief description of the error, including primary relevant information. The detail may be a longer discourse providing further information about the cause of the error and/or possible remedies. For example,
  message:
    "Unable to open the file 'InterestingStuff.txt'".
  detail:
    "Someone has spread peanut butter on the disk. Please clean it and try again."

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