api: trim space of error response output

[shoenig]

This PR does a strings.TrimSpace on the response body of a failed API request, so that an error response is not extended with unwanted whitespace.

before:

➜ nomad job run bad.nomad
Error submitting job: Unexpected response code: 500 (1 error occurred:
	* Task group cache validation failed: 1 error occurred:
	* Task group service validation failed: 1 error occurred:
	* Services are not unique: [group->foo]





)

after:

➜ nomad job run bad.nomad
Error submitting job: Unexpected response code: 500 (1 error occurred:
	* Task group cache validation failed: 1 error occurred:
	* Task group service validation failed: 1 error occurred:
	* Services are not unique: [group->foo])

[shoenig]

Bonus update of golangci-lint because maybe it fixes the depguard failure 🤔

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