Create a better error message for unified backend failures

[umar456]

The unified backend loads several libraries and its difficult to figure out what is the cause of the failure. Th

ERROR app - ArrayFire Exception (Failed to load dynamic library. :501):
Error in af_err unified::AFSymbolManager::setBackend(af::Backend)
In file src/api/unified/symbol_manager.cpp:239
Failed to load dynamic library. See http://www.arrayfire.com/docs/unifiedbackend.htm for instructions to set up environment for Unified backend.

We should return an error message that describes the source of the failure. af_get_last_error should return this message and the exception should have this set as the message.

[9prady9]

@umar456 With stack trace, trace output showing which backend library failed to load. Can we save this is complete or are we still missing something ?