Support being used to supplant `opam install`, even in a non-opam directory

[ELLIOTTCABLE]

opam otopop should probably be a full superset of opam install — or at least, should be for simple package-names with no qualifications; and should be when there's no .opam file to modify. This interaction, for example, surprised me:

$ opam otopop menhir
[ERROR] No opam file in /Users/ec/Sync/Code/wrange

[ERROR] opam install ended with a non-zero exit code:
        internal error, uncaught exception:
                     OpamStd.OpamSys.Exit(5)

[rjbou]

This plugin main purpose is to populate the opam file while wrapping the install, simple packages names are handled. Here, it is the error catching that is misleading: the error is raised by the plugin while it displays that the install failed. Before performing the install, the opam file to populate is checked.

If the case where just an install is needed, why don't use directly opam install?

[rjbou]

added better error handling & reporting in 5ec024b