PASTCAST wxModel needs more robust error checking, for GUI

[latwood]

When an invalid time is used, PASTCAST acts like a file is being properly downloaded, when an error should be thrown. Examples of invalid times are when a time is too recent, or when the startTime is later than the endTime.

When a PASTCAST fails in general, it acts like a file is being properly downloaded, instead of throwing an error.

Because error messages aren't even made, let alone thrown, the GUI fails to properly end the PASTCAST fetching.