Watchexec Files

Fixes a panic (at CLI construction, so it's really more a UI cleanup):

$ watchexec --shell=notashell -1 -- echo hi
thread 'main' panicked at crates/cli/src/config.rs:1268:5:
  The parsed shell path "notashell" does not exist on this system, and could not be resolved from your PATH.
  note: run with RUST_BACKTRACE=1 to display a backtrace

thanks @hktitof (#1105)