Fix relup cmd arguments and add validation

[garazdawi]

There seems to be an intention in the relx:build_relup code to support Release, ToVsn and FromVsn to be undefined, but that does not work nor does the spec allow for it. So this PR fixes rebar_relx to work with --relname and --relvsn and informs the user that you have to give these options.

Another option would be to also fix relx:build_relup/4 to allow Release, ToVsn and FromVsn to be undefined to be undefined.

[tsloughter]

Thanks, this looks good. Currently we keep tests of the cli separately in relx itself https://github.com/erlware/relx/tree/master/shelltests and a separate repo that runs tests nightly, https://github.com/tsloughter/rebar3_tests

So I'll get a tested added to one of those, unless you feel like it :)

[garazdawi]

See tsloughter/rebar3_tests#3 for test

[jwheare]

I believe this breaks a pattern we were using where the relname and relvsn are passed along when running the chained command: rebar3 release relup -u <upfrom>

[tsloughter]

Chained commands don't work like that anymore. Do you mean, rebar3 do release, relup -u <upfrom>?

[jwheare]

Hmm, I think there are actually quite a lot of things that have changed that break our setup. I've rolled back to the version we were on before (3.9.1). Will try to dig into this some more another time.