Improve error messages during RDF translation about transforge

[nsbgn]

The commits over the past few days, addressing the improvement of the textual representation of our data structures (4349a30, 5bd551c, a0a9fb9, bb61697), type-annotated parsing (9b83455, e144146) and enabling more succinct test cases (4487c79, b75f9f1, 994d31a) all had as goal to create order in the chaos.

The next step is to move errors to the modules that are relevant to them.

Also, instead of clumsily annotating the expressions in which type errors occur, we can wrap a type error in an expression error. We can still test which type error is raised by inspecting the __cause__ attribute set when you use raise from (see https://stackoverflow.com/questions/24752395/python-raise-from-usage). For unittesting, we can write an assertRaisesChain that wraps assertRaises used as a context manager (see https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaises).

This approach also allows us to raise another wrapped error for errors that occur during the primitive expansion of an expression containing a composite operator.

[nsbgn]

More thoughts:

• Every relevant class (Operator, TypeSchema) gets a method simply called validate() that gives explicit guarantees as to what assertions the user may make if it completes without error.
• There should be a WorkflowError exception that tells you on which attachment of input and output an error occurred.
• visual_test files should be present that aren't for unit testing but just for checking how the errors look when they are raised.

[nsbgn]

Current issue can be closed with 1e232ab. visual_test files aren't made yet but that's an idea for later.