Add toolchains to dependency error messages in GN

This adds toolchain labels to the error message about depending on generated
files if any of the toolchains are not in the default toolchain. The toolchains
can be confusing (normally they're all in the default one) but for
cross-toolchain dependencies, the error message becomes impossible to
understand without them.

Review URL: https://codereview.chromium.org/1270603004

Cr-Commit-Position: refs/heads/master@{#341221}
1 file changed