Fix colourised output in error messages

This fixes a small mistake in 4dc681c7 which forced the dump rather than user style for error messages.

In particular, this change replaced defaultUserStyle with log_default_dump_context rather than log_default_user_context which meant the PprStyle was PprDump rather than PprUser for error messages.

https://gitlab.haskell.org/ghc/ghc/-/commit/4dc681c7c0345ee8ae268749d98b419dabf6a3bc?expanded=1&page=4#b62120081f64009b94c12d04ded5c68870d8c647_285_405

Fixes #20276 (closed)