hugolib: Make missing GitInfo a WARNING
hugolib: Make missing GitInfo a WARNING

And not an ERROR.

Fixes #3376

fedelibre/hugo,gohugoio/hugo,fedelibre/hugo,fedelibre/hugo,anthonyfok/hugo,mdhender/hugo,mdhender/hugo,mdhender/hugo,gohugoio/hugo,fedelibre/hugo,anthonyfok/hugo,mdhender/hugo,gohugoio/hugo,digitalcraftsman/hugo,mitchchn/hugo,anthonyfok/hugo,mitchchn/hugo,digitalcraftsman/hugo