fix(macros): slightly improve unsupported type error message

[dyc3]

Does your PR solve an issue?

This PR makes a small improvement to the "unsupported type" error message to suggest a potential solution to the user.

I just spent a couple hours on an issue forgetting that type overrides exist and I wished that the message nudged me in that direction.
Perhaps the message could be made even more intelligent.

Is this a breaking change?

No. It's a minor DX improvement.

[abonander]

I've significantly revised the wording as part of #3383:

• parameter types (though the default fallback message is the same--I need to fix that): https://github.com/launchbadge/sqlx/pull/3383/files#diff-4699a1c09e40cbc3a05ed9fcb9849dbb318725d0d9c4e30ad4e5fa73c03022caR126-R161
• output column types: https://github.com/launchbadge/sqlx/pull/3383/files#diff-1134d3dad8c37a9fc57c01ce7ebea40a264c872651ba7818c9eeb2f5de69d345R271-R330

There's some significant changes coupled in there, but if you'd like to backport the wording, I wouldn't be against it.

[dyc3]

Ah, so should I make this PR against that branch instead?

[abonander]

No, I meant that you could copy the wording changes I made in that PR to this one, since that's slated for 0.9.0.

Probably going to have one more patch release before 0.9.0 since we've accumulated a few regressions, so that part could be backported.

[dyc3]

Oh, ok. Yeah I can do that.

[dyc3]

This should be ready to go. Let me know if I should do more.