Make MatrixError a non-default export

[Half-Shot]

I made the mistake of making this class a default, which means that when it was exported it would not be a named class and therefore be unimportable. We've worked around this in some projects by directly depending on MatrixError.ts file, but that's a sucky way to do it.

This PR changes the class to be a regular exported class, and changes all internal references to match. This will break projects that depended on the class being a default, but arguably that's not supported behavior. It's also trivial to go and fix your imports on the next update.

Checklist

• [N/A - types change] Tests written for all new code ()
• Linter has been satisfied
• Sign-off given on the changes (see CONTRIBUTING.md)

Signed-off-by: Will Hunt signoff@half-shot.uk