subrepo: better error messages in _ensuregit This patch improves the error messages raised when an OSError occurs, since simply re-raising the exception can be both confusing and misleading. For example, if "hg identify" is run inside a repository that contains a Git subrepository and the git binary could not be found, it'll exit with the message "abort: No such file or directory". That implies "identify" has a problem reading the repository itself. There's no way for the user to know what the real problem is unless they dive into the Mercurial source, which is what I ended up doing after spending hours debugging errors while provisioning a VM with Ansible (turns out I forgot to install Git on it). Descriptive errors are especially important on Windows, since it's common for Windows users to forget to set the "Path" system variable after installing Git.