Vet --os and --arch args and improve error messages for bad inputs

[sjakobi]

~/s/path (master) $ stack test --os foo
Compiler version mismatched, found ghc-7.6.3 (x86_64), but expected minor version match with ghc-7.10.3 (x86_64) (based on resolver setting in /home/simon/src/path/stack.yaml).
Try running "stack setup" to install the correct GHC into /home/simon/.stack/programs/x86_64-foo/
~/s/path (master) $ stack test --arch foo
Compiler version mismatched, found ghc-7.6.3 (x86_64), but expected minor version match with ghc-7.10.3 (foo) (based on resolver setting in /home/simon/src/path/stack.yaml).
Try running "stack setup" to install the correct GHC into /home/simon/.stack/programs/foo-linux/
~/s/stack (master) $ stack --version
Version 1.1.3, Git revision 11f225d126b6ec3cd56d7d45e830540ee82dca4f (dirty) (3720 commits) x86_64 hpack-0.14.1

The error message for --os foo is rather unhelpful. I'm also not sure why it chose this system-installed ghc-7.6.3?

Also it would be good to emit a warning when the OtherOS or OtherArch constructors are used (see Cabal haddocks.

[mgsloan]

Gnarly, good catch!

[sjakobi]

It would probably be better to address #2227 before any work is invested in "improving" --os.

[mgsloan]

Fixed for --arch, says:

Warning: Unknown value for architecture setting: "foo"