cmd/dist: fails w/o .git or VERSION with no actionable error message

[proppy]

Building a tarball snapshot fails with the following error, because the source tree is not a git repo:

...
##### Building compilers and Go bootstrap tool for host, darwin/amd64.
++ buildall=-a
++ '[' --no-banner = --no-clean ']'
++ ./cmd/dist/dist bootstrap -a -v
fatal: Not a git repository (or any of the parent directories): .git

go tool dist: FAILED: git rev-parse --abbrev-ref HEAD

An "easy" workaround is to create a VERSIONfile, it would be nice if the error message was pointing that out.

[bradfitz]

Yeah, a nicer error message would be good there.

/cc @adg