All the call sites of `variable_ident_decl()` add their own error handling, so transform this into `expect_variable_ident_decl()`.
Also makes error messages more consistent.
Bug: tint:282
Change-Id: I0b5ac984018ba78896ddec0320636f5b5c4ad0b2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/32100
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>