rhg: Add a `rhg.on-unsupported` configuration key...
rhg: Add a `rhg.on-unsupported` configuration key
For now the two values are:
* `abort-silent`: silently exit with code 252, the previous default behavior
* `abort`: print an error message about what feature
is not supported, then exit with code 252. Now the default.
Differential Revision:
https://phab.mercurial-scm.org/D10091