Feature request: GMML error generator

[danwentworthart]

The problem: GMML devs are good, so they fix problems quickly, leaving small opportunities for improving error handling.
Another problem: In dev, it can be a challenge to know all the error types and contexts that may arise, until they actually happen.

The solution: A way for GMML to give GEMS an error on purpose. GEMS would need to know how to find these, and package them in json responses that the website knows how to read and display.

The ability to cause errors during tests can ensure that our error handling is tested as well, at all levels.

[gitoliver]

Sequences and the errors thrown by the carb builder are shown here:
https://github.com/GLYCAM-Web/gmml/blob/actual/tests/tests/correct_outputs/023.output_carbohydrateBuilder.txt
Note that not all of them throw an error, some build.

Look for Test level caught exception in these ones, these are been thrown by the sequence class:
https://github.com/GLYCAM-Web/gmml/blob/actual/tests/tests/correct_outputs/015.output_sequenceAssembly.txt

Carb builder tries to build a 3D structure, so the sequence might be fine it's just that we don't have metadata. The sequence class is checking that the sequences conform to our Glycam nomenclature(s).