Improved error checking code and fixes to code for system freezes.

[jcp-sd]

The important changes are to mbox_open() and open_mbox(). The remaining changes are not required but should greatly improve error handling should similar issues appear in the future.

[Supremehamster]

Thanks. Solves my crashing problem on the Pi2b.

[jcp-sd]

Hi James,

I'm just guessing you have been very busy and didn't notice my PiCW pull request. Could you merge in my changes when you get a chance? (They are virtually identical to the ones I put together for WsprryPi around the same time).

There have been some recent posts to the TAPR HF-SIG from people having problems with your branch of PiCW that sound pretty similar to the problems I was having.

Thanks, John