Give proper error messages when parsing improper AtlasImport file.

[hanjoosten]

For the Atlas import, a JSON file is generated by the Atlas. However, there can be issues with the generated file. I.e, the name of a concept cannot be any string. It must abide to the rules for a name. Currently, when this happens, a fatal is thrown. The JSON parser should be refactored, so it doesn't result in a P_Context, but in a Guarded P_Context.