vcs_tag() error output should be silenced if fallback is specified

meson's vcs_tag function is very useful but the fact that it always generates error output if no annotated tag matching the current git revision is found is... unnecessary.

We use actual git revisions as our version numbers for intermediate releases, so we provide the output of git rev-parse HEAD as the value of the fallback parameter to vcs_tag. That works great, but each time meson complains about no annotated tags:

ninja -C debug
ninja: Entering directory `debug'
[1/4] Generating GitVersion.cpp with a custom command.
fatal: No annotated tags can describe 'e5c806c6a0fc0380637b955b018485ccd1214ddd'.
However, there were unannotated tags: try --tags.

If a fallback is provided, I think that output should be silenced.