add underlying cause for TransportError

[gchux]

Is it possible to save a reference to the last exception thrown at:

google-auth-library-python/google/auth/compute_engine/_metadata.py

Line 204 in 0afc61a

response = request(url=url, method="GET", headers=headers_to_use)

and use it to build a more meaningful error message? at:

google-auth-library-python/google/auth/compute_engine/_metadata.py

Lines 217 to 220 in 0afc61a

	raise exceptions.TransportError(
	"Failed to retrieve {} from the Google Compute Engine "
	"metadata service. Compute Engine Metadata server unavailable".format(url)
	)

that way the underlying reason for the TransportError would be available even if DEBUG logging level is not enabled.

[clundin25]

Should be a small and easy fix. Just add a variable store the last seen exception, and inherit it using the from syntax.

https://docs.python.org/3/tutorial/errors.html#exception-chaining