gracefully report un-parsed upstream URL

[ploxiln]

fixes #430

Current master branch:

$ ./oauth2_proxy --upstream=127.0.0.1:8080
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x13b8d2f]

goroutine 1 [running]:
main.(*Options).Validate(0xc4200f8000, 0xc4200f8000, 0xc420012480)
	/Users/pierce/go/src/github.com/bitly/oauth2_proxy/options.go:149 +0x30f
main.main()
	/Users/pierce/go/src/github.com/bitly/oauth2_proxy/main.go:101 +0x1303

This branch:

$ ./oauth2_proxy --upstream=127.0.0.1:8080
2017/08/05 13:17:42 main.go:103: Invalid configuration:
  missing setting: cookie-secret
  missing setting: client-id
  missing setting: client-secret
  missing setting for email validation: email-domain or authenticated-emails-file required.
      use email-domain=* to authorize all email addresses
  error parsing upstream: parse 127.0.0.1:8080: first path segment in URL cannot contain colon

cc @tpherndon

[jehiah]

👍 much more useful!