Fix status code when bad command name is entered

[radomirbosak]

Description

This commit fixes a bug which causes that

fish -c ')'; echo $status

("Illegal command name" error) returns 0. This is inconsistent with
e.g. when trying to run non-existent command:

fish -c 'invalid-command'; echo $status

("Unknown command" error) which correctly returns 127.

A new status code,

STATUS_ILLEGAL_CMD = 123

is introduced - which is returned whenever the 'Illegal command name *'
message is printed.

This commit also adds a test which checks if valid commands return 0,
while commands with illegal name return status code 123.

Fixes #3606.

TODOs:

• Changes to fish usage are reflected in user documenation/manpages.
• Tests have been added
• Decide if new status code STATUS_ILLEGAL_CMD=123 should really be added, or STATUS_UNKNOWN_COMMAND=127 should be reused (as the issue reporter suggested)

What do you think? Should STATUS_UNKNOWN_COMMAND be used instead? I don't have problems rewriting the commit to use it instead of new status code.

And please check the test. I'm not very experienced with C++ .

[ridiculousfish]

Thanks for the great contribution. The tests look outstanding!

I like the introduction of STATUS_ILLEGAL_CMD since that seems like a very distinct error from STATUS_UNKNOWN_CMD. The first is for syntactic errors, the second for commands that were not found.

[ridiculousfish]

Merged. Thank you!!