If repo init is run with the --mirror option, repo checks if there
is already a workspace initialized in the current location, and if
so, exits with an error message:
--mirror not supported on existing client
This error can cause confusion; the users do not understand what
is wrong and what they need to do to fix it.
Change the error message to make it a bit clearer.
Change-Id: Ifd06ef64fd264bd1117e4184c49afe0345b75d8c