silence usage messages when usage was valid and API errors are returned

[displague]

Silence usage errors to make API errors more apparent. Currently, the API Usage messages can easily hide API errors.

See spf13/cobra#340 (comment) for implementation alternatives. The linked comment is the approach that I found to be most effective.

A few functions were refactored to get usage validation out of the way before silencing usage rendering and executing API commands. Some of these functions had condition branches that could be logically hoisted out of else conditions.

Before:

$ go run ./cmd/metal/main.go  capacity check --metro xy -P c3.small  -q 1
Error: Could not check capacity: POST https://api.equinix.com/metal/v1/capacity/metros: 422 Metro xy is invalid
Usage:
metal capacity check [flags]
... 20 more lines of text

and After:

$ go run ./cmd/metal/main.go  capacity check --metro xy -P c3.small  -q 1
Error: Could not check capacity: POST https://api.equinix.com/metal/v1/capacity/metros: 422 Metro xy is invalid 

When invalid argument combinations are offered, the usage text is rendered.

$ go run ./cmd/metal/main.go  capacity check --metro xy -P c3.small 
Error: required flag(s) "quantity" not set
Usage:
  metal capacity check [flags]