Fail step context list when there are no contexts

[arpitjain099]

Fixes #879.

Still reproduces on master:

$ export STEPPATH=/tmp/steppath   # empty
$ step context list
$ echo $?
0
$ step context current
no context selected
$ echo $?
1

listAction returns nil unconditionally, so an empty STEPPATH looks exactly like a successful listing that happened to have nothing in it. step context current already errors in the equivalent situation, which is what @jdoss ended up using as the workaround, so this makes list behave the same way.

After:

$ step context list
no contexts present
$ echo $?
1

The message goes to stderr (stdout is empty with 2>/dev/null), which is what the issue asked for.

I checked the populated case too, since the listing loop now runs over a variable rather than calling ListAlphabetical() inline:

$ STEPPATH=/tmp/steppath2 step context list
▶ beta
alpha-one
$ echo $?
0

Byte-identical to master's output for the same STEPPATH.

No test here: listAction reads the package-level step.Contexts(), which needs step.Init() and a STEPPATH to be set up, and nothing under command/ currently does that, so a test for this one command would have meant introducing global-state setup that has no precedent in the tree. Happy to add one if you would like it, and if you have a preferred way of standing up a STEPPATH in tests I would rather follow that than invent one.

go build ./..., go vet ./command/context/ and go test ./command/... are clean.