OpamSystem.resolve_command should display better error when permissions not set correctly

[dra27]

The bottom of this log includes:

Processing  5/6: [graphics: ls install.sh]
+ /bin/ls "-l" "install.sh" (CWD=/home/opam/.opam/4.07/.opam-switch/build/graphics.4.07.1)
- -rw-r--r-- 1 opam opam 1980 Nov  1 18:57 install.sh
Processing  5/6: [graphics: ocaml unix.cma]
+ /home/opam/.opam/4.07/bin/ocaml "unix.cma" "probe.ml" (CWD=/home/opam/.opam/4.07/.opam-switch/build/graphics.4.07.1)
- uid=1000  groups=  stat=1000,1000,644

#=== ERROR while compiling graphics.4.07.1 ====================================#
"./install.sh": command not found.

In this case, the command is found but, as the crude test above reveals, it is not executable. The permissions check in OpamSystem.resolve_command should more clearly state that the execute bit is missing completely (as here) or that the command is not executable to the current user (if u+x or g+x is specified but the scripts uid/gid do not match the process's)