Organise exceptions better in codebase

[mr-smidge]

Improve the way that exception classes are organised in the codebase:

1. Low-level Engine API exceptions should all derive from an abstract base class engine_exception or similar.
2. High-level exceptions should all derive from an abstract base class djinterop::exception or similar.
3. Exceptions should be defined in the header containing declarations that might throw them, not in exceptions.hpp.
4. Exceptions referencing numeric ids should have that parameter removed, with issue Consider track/crate "id" struct suitable for high-level API #93 in mind.