The first commit in this pull request catches an exception caused when the HTF class tries to use an invalid custom HTF table. Now the UI complains, because it isn't getting real values from the callback, but it doesn't crash.

The callback on the Physical Trough Solar Field page isn't setting the fluid temperature limits. Previously this happened through a UI function, like how the Phys Trough Solar Field page currently sets the limits. (The IPH model uses the Physical Trough Solar Field page, but its old UI page still exists and includes the temperature limits...) @Matthew-Boyd do you want to migrate the temperature limits to SSC?

@cpaulgilman Do we want to disable the Edit button when a library HTF is selecting? With the currently problems with invalid custom HTFs, it might be nice to be able to edit the HTF properties before selecting the custom HTF in the drop down.

Also needed to catch the HTF exception for the MSPT callback. The MSPT UI disables the Edit button.

@cpaulgilman Do we want to disable the Edit button when a library HTF is selecting? With the currently problems with invalid custom HTFs, it might be nice to be able to edit the HTF properties before selecting the custom HTF in the drop down.

The SAM custom-htf branch disables the Edit button (MaterialProperties) for all CSP models unless the "user-defined" option is selected for the HTF. I think this makes it clear that you can only edit custom HTF properties. If we don't disable the Edit button, people might think they can edit the properties of the non-custom HTFs in the list.