Error handling option

Hi, thanks for sharing this, very handy.

We currently have a slightly curious setup with devs using either Subversion or Git SVN to share code internally. I've got `grunt-svninfo` working for the SVN people, and I'd like something in place for those using Git... but `git-describe` is would currently break the build for those not using Git.

So... here's a pull request that leaves the default behaviour unchanged, but adds a failOnError option, that if you explicitly set to `false` will allow the build to continue on an error (such as git not being found). Hopefully I've stuck closely enough to the existing style. I've also documented the new option in the README.

I haven't added tests because I didn't want to push my test-framework choices onto you, but happy to add tests for this later if you decide on test tools