-S: error out if set and no swap is available
-S: error out if set and no swap is available

Should fix the test failure on Travis:

	cli_test.go:99: Testcase #17: earlyoom -S 9999999999999999
	cli_test.go:103: wrong exit code: have=-1 want=16
	cli_test.go:107: stdout should be empty but is not
	cli_test.go:119: stderr should contain "above total swap", but does not
	cli_test.go:130: stderr:
		earlyoom v1.1-20-gf585a6f
		mem  total: 7479 MiB, min: 10 %
		swap total:    0 MiB, min:  0 %
		Could not lock memory - continuing anyway: Cannot allocate memory
	cli_test.go:131: stdout:
		mem avail: 6835 of 7479 MiB (91 %), swap free:    0 of    0 MiB ( 0 %)
	cli_test.go:99: Testcase #18: earlyoom -r 0
	cli_test.go:99: Testcase #19: earlyoom -r 0.1
	FAIL

https://travis-ci.org/rfjakob/earlyoom/builds/404207880

rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom