stop using errors.cause, simply return the error itself

[jmuk]

Based on googleapis/google-cloud-node#1721 (comment):

There might not be a clear benefit of using 'Error' with cause property in NodeJS. This design originally came by respecting the design of gax-python, and that's good for Python because it fits with try...catch pattern.

gax-nodejs doesn't expect try...catch pattern at all, and we always use bare Error, thus it turns out this design does not fit to the actual usage quite well. Let's remove this.