connector: Doc the errors of NewAWSReader

[ifraixedes]

When using this service and wanted to actually know which errors can be returned by the NewAWSReader, the implementation of it had to be checked. For avoiding of having to do it each time by whoever needed it, the errors that the NewAWSReader have been documented.

This PR has documented some internal functions because it was the way to track from the bottom-up which errors the NewAWSReader can return, it was not necessary for the end purpose, but having such docs are more helpful than harmful.

[coveralls]

Coverage remained the same at 84.615% when pulling b47d350a7c4719e3369849f5b3af1f4e142e8848 on if-endow-err-doc into c8aea0353ae0a774c6c5d3d844e33a519f8b0b12 on master.

[xescugc]

You can merge it :+1: (it's approved already hehe)