`exec` incorrectly reports syntax error

[mqudsi]

When exec fails because the process cannot be found, the default command-not-found handler is automatically executed. However, fish also emits a blank syntax error message, pointing to the name of the process that can't be launched:

mqudsi@Blitzkrieg /m/d/r/fish> exec fg
fg: command not found
fish:
exec fg
     ^

I believe the output should simply be

~> exec fg
fg: command not found
~> 

[faho]

Fixed in master by 3534c07.