Misleading error message in `exec-path/env'

The procedure `exec-path/env' gives "No executable found." if it fails
to execute the given program.  This error message is misleading since
there can be other reasons for the failure, e.g. the command line was
too long.  I think something along the lines of "Failed to run
program/executable." would be more appropriate.

-- 
Emílio C. Lopes
Munich, Germany