Catch 'user not found' type of errors

[dokterbob]

When requesting a token for unknown users, iProva yields the following exception:

WebFault(u"Server raised fault: 'System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.Exception: The user is not known in the system!\n   at Infoland.Suite.Security.GetTokenForUser(String strTrustedApplicationID, String strLoginCode, HttpRequest objRequest)\n   --- End of inner exception stack trace ---'",)

The proper way to handle this (and similar events) is to create a RequestException subclass UserNotFoundException and do a string match on the error string for the aforementioned exception. This allows for much more precise logging and discrimination of 'user not found' events (which are relatively harmless) from more severe errors.