Defining a command without cmd or valid imports defined causes panic

[frankcarey]

This should fail more gracefully with a helpful error message.

Example files:

version: v2
commands:
  example:
    usage: This doesn't have a command defined.

version: v2
commands:
  example:
    usage: 'cmd' is misspelled as 'dmc'
    dmc: echo "test"

version: v2
commands:
  example:
    usage: This doesn't have an import where the file actually exists.
    imports:
      - file-not-exists.yml

version: v2
commands:
  example:
    usage: This has 'imports' misspelled as 'import'
    import:
      - import-exists.yml

Panic Error

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x74918]

goroutine 1 [running]:
panic(0x17d220, 0xc4200120b0)
	/usr/local/Cellar/go/1.7.3/libexec/src/runtime/panic.go:500 +0x1a1
github.com/ahoy-cli/ahoy/vendor/github.com/codegangsta/cli.Command.Run(0xc4200705a0, 0x6, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc4200705c0, 0xf, 0x0, ...)
	/private/tmp/ahoy-20161121-46993-73ob16/src/github.com/ahoy-cli/ahoy/vendor/github.com/codegangsta/cli/command.go:131 +0x798
github.com/ahoy-cli/ahoy/vendor/github.com/codegangsta/cli.(*App).Run(0xc42009e120, 0xc420084060, 0x2, 0x2, 0x0, 0x0)
	/private/tmp/ahoy-20161121-46993-73ob16/src/github.com/ahoy-cli/ahoy/vendor/github.com/codegangsta/cli/app.go:175 +0xa35
main.main()
	/private/tmp/ahoy-20161121-46993-73ob16/src/github.com/ahoy-cli/ahoy/ahoy.go:379 +0xea

[frankcarey]

This now has errors and tests specific to each case