print subprocess error in mirror

[YangKeao]

Enhancement

The cmd/mirror doesn't print error messages to help debug the problem. For example, what we got is:

panic: exit status 1

I expect:

panic: subprocess exited with stderr: 
go: github.com/twmb/murmur3@v1.1.6: missing go.sum entry for go.mod file; to add it:
        go mod download github.com/twmb/murmur3