[mojo] Enhance the StructTraits failed-to-find assert message.

Before this change the compile error that catches missing StructTraits
specialization suggests confirming the header file is included properly
as a potential fix. However, the build system automatically includes
the header so it's a little confusing as to what this is suggesting
might be wrong.

After this change we specifically suggest the reader to check their
build config to confirm the header is there and to confirm the
StructTraits specialization is defined as expected.

Bug: N/A
Change-Id: I57d6eb8695fd4d21038f105b444c3ddde3205dcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6349118
Reviewed-by: Fred Shih <ffred@chromium.org>
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1431767}