Better error message for failed app link

[lornajane]

Steps to reproduce

nexmo app:link [number that I own] [wrong app ID]

Expected behavior

Error message saying "couldn't link to that app" or something.

Actual behavior

Error message: method failed

Same error message in verbose mode

System configuration

Node version: v9.3.0

NPM version: 6.1.0

Operating system: Ubuntu 18.04

(I'm happy to attempt a patch if anyone else is happy for me to do that)

[mheap]

Sadly there's no easy way to do this as it comes from the API. See https://github.com/Nexmo/nexmo-php/blob/master/src/Numbers/Client.php#L229-L238 for more information