More reductions in error handling diversity

[Centril]

In this follow up to #67744, we:

• Remove all fatal / error / warning macros in syntax except for struct_span_err, which is moved to rustc_errors.

• Lintify some hard-coded warnings which used warning macros.

• Defatalize some errors.

In general, the goal here is to make it painful to use fatal or unstructured errors and so we hopefully won't see many of these creep in.

Fixes #67933.