My last commit checked for missing executables before trying to execute
them, but still left open the possibility that it would be impossible to
call exec(2) on an executable that had just been found (for whatever
reason).

Closer inspection in 'perldoc perlipc', and 'perldoc -f open' has helped
illustrate how to catch that condition. E.g. it turns out that calling
'close' on a pipe-opened filehandle will automatically call waitpid,
plus I'd forgotten to exit in that error case in the child proc. (This
is why it would take 2-3 seconds after failing for the parent process to
continue).

The more interesting question is what error policy to use in this case.
Errors are expected in pretend mode (this function actually runs
processes even during pretend mode since otherwise it can be impossible
to determine whether to clone or update, switch branches, etc.), but on
the other hand during execution errors should be unexpected and
noteworthy.

FIXED-IN:15.04
BUG:345425