Improve selection error messages (use exceptions)

Improve selection error messages (use exceptions)

Make all selection parsing error handling use exceptions for error
reporting, and improve the understandability and context-sensitivity of
the messages.  Make the code in params.cpp exception-safe.

Resolves #655 (except for some pending cleanup of now-unused code).

Change-Id: If7bc1da91041232db23389c660cc2cd9c50addda