rhg: Remove `rhg.fallback-executable=hg` default configuration When `rhg.on-unsupported` is configured to `fallback` and an unsupported feature is encountered, the previous default was to look for an `hg` executable in `$PATH`. This default was fragile since it was easy to end up accidentally using an older version of Mercurial installed system-wide, when a local (perhaps patched) installation was intended. Instead, it is now an error to have `rhg.on-unsupported=fallback` without also configuring an explicit path or the fallback executable. Differential Revision: https://phab.mercurial-scm.org/D10189

rhg: Return an error code for `rhg config Section.idontexist` This is what Python-based hg does. Differential Revision: https://phab.mercurial-scm.org/D10145

rhg: Fall back to Python for --version Clap has some built-in support for printing something on --version, but it looks different than what Pytho-based hg does. Also, at the moment we’re not giving version numbers to rhg separate from the Mercurial version. Differential Revision: https://phab.mercurial-scm.org/D10144

rhg: Exit with an error code if `files` finds nothing This matches the behavior of Python-based hg. Differential Revision: https://phab.mercurial-scm.org/D10143

rhg: `cat` command: print error messages for missing files And exit with an error code if no file was matched. This matches the behavior of Python-based hg. Differential Revision: https://phab.mercurial-scm.org/D10142

rhg: Silently ignore missing files in config %include … instead of aborting with an error message. This is what Python-based hg does. Differential Revision: https://phab.mercurial-scm.org/D10141