Fix error message when no private key is provided

[seveas]

Short description

This error had me confused for a minute. Had to read this file to realize that I passed an empty string as PrivateKeyPath

Checklist

• [x] I read the CONTRIBUTING.md document
• [x] This code actually compiles
• [x] This code solves my problem
• [ ] I've updated the inline documentation
• [ ] I added or modified test(s) to prevent this issue from ever occuring again

Last 2 are not really applicable 😃

[dionbosschieter]

Hi @seveas,

This will make the test expecting that error also fail. Could you also change that test?

--- FAIL: TestNewClient (0.00s)
    client_test.go:48: 
                Error Trace:    client_test.go:48
                Error:          Error message not equal:
                                expected: "PrivateKeyReader, token or PrivateKeyReader is required"
                                actual  : "PrivateKeyPath, token or PrivateKeyReader is required"
                Test:           TestNewClient
FAIL

[seveas]

Certainly! Apologies for the sloppy pr.