Additional try/catch for C API

[guruofquality]

This PR represents additional try/catch blocks for C API device calls. This causes graceful returns with error message set (vs std::terminate when the uncaught exception throws in C). The API was not properly set up to provide error return codes. We are not introducing any C API changes in this commit. This is a fix commit for 0.5 series; so the following was done for existing calls:

• All errors thrown set SoapySDRDevice_lastError()
• API calls that returned points will return NULL on error
• API calls that return PODs return -1 or NAN (whatever fits)
• API calls that return void set the error, but there is nothing to show
• Because error cannot be determined for all return types, a int SoapySDRDevice_lastStatus() is provided to determine the error state of the last device API call.

Related issue: #95