Panic when command does not exist

[Stavrospanakakis]

To Reproduce
Steps to reproduce the behavior:

1. Run is_ready with a command that does not exist. For example:

is_ready --addr google.com:80 -- command_that_does_not_exist

2. It will panic:

Connected to google.com:80 successfully
thread 'main' panicked at is_ready/src/lib.rs:73:10:
Failed to run the command.: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expected behavior
In this case, it should handle the error and return something more user friendly such as Command not found