This updates djblets.integrations.errors to include type hints and to

add some docstrings that were previously missing.
It also fixes an issue with using IntegrationNotRegisteredError. This

could be called with a None integration ID, which produced a poor

error message. To avoid introducing a new error and affecting consumers,

this class now provides a more specific error message with a None ID.

Tested along with consumers and suppliers of this error.
Unit tests pass.