SIGABRT when trying to read invaild metadata

[Aron Szabo]

LibSigMF should throw an exception when dealing with a SigMF metadata file with invalid field structure. Calling sigmf::metadata_file_to_json on such a file aborts the program, with no way to handle the error. The assertion fails when operator[] is called on a non-existent JSON key.
When the JSON itself is invalid, an exception is generated in json::parse, and the software can recover from this situation correctly.
How to reproduce: change one letter in the key of a required field in the sigmf-meta file
Expected behaviour: Some kind of exception
Actual behaviour: SIGABRT