Improved error handling for missing git config

This will set name / email to undefined if .gitconfig does not exist.

Currently if the values are not set shell.exec will return an empty string and the line ```this.user.git.username || process.env.user || process.env.username,``` from generator-generator's app generator will always return empty string.

If this pull is accepted generator-generator will need to be updated to handle this.user.git.email returning ```undefined``` or you might want to only include the error handling for name