Add an error print if dlopen fails

[trws]

Originally from #236.

I'm not sure this is the best way to handle it, but otherwise the error for a
module dependency being unavailable is "invalid argument" in the broker. This
has to be more debuggable, but I'm open to suggestions for better handling
here.

[grondo]

Hm, yeah, this error really needs to be propagated back to the caller (ideally all the way back to the client using flux-module insmod (?)).

There is a similar error in flux_modname_cmp which should probably be fixed in this same PR (not sure if that one causes an error in practice though)

Ideally flux_modname would somehow be able to send a useful error back to caller, but maybe in general the interface should support grabbing a modinfo handle that stores module name and service or error information if path can't be opened:

e.g.

flux_modinfo_t *mi = flux_modinfo_get (path);
if (!mi)
    /* out of memory */
if (flux_modinfo_errmsg (mi))
   /* return errmsg somehow */
service = flux_modinfo_service (mi);
name = flux_modinfo_name (mi);

out_cleanup:
flux_modinfo_destroy (mi);

Not sure if this is the best way, just thinking aloud. I though Jim had a pretty good design for the loadable module support, so perhaps propagating these errors is covered somewhere.

[garlick]

Merging, maybe a better way can come later.

[garlick]

Oops actually @trws perhaps you could squash the two commits and rebase on current master first?

[trws]

No problem, should be all set now @garlick.