Error if home directory isn't writable

[PeterJCLaw]

I've recently noticed that pandas_gbq requires that a user's home directory exist and be writeable in order to be used. This means that it cannot be used in restricted environments, where that may not be the case.

I think this is a result of the recent change (#241) to use https://github.com/pydata/pydata-google-auth for auth.

I've raised the underlying issue as pydata/pydata-google-auth#10, however from what I can tell pydata_google_auth is only actually used in the case where the user doesn't provide credentials as part of an operation.
In particular, I'm using a service account (where you're more likely to be in a restricted environment too), and always provide the credentials upfront anyway.

It would therefore be great if pydata-google-auth was a soft requirement rather than a hard one -- both removing it from the requirements of this library and from the checks in _test_google_api_imports, or making those checks aware of whether credentials have been explicitly provided.

[tswast]

Interesting point that we could make this a soft requirement. I think we could probably delay import time until the library is needed (for example, when credentials are not provided).

That said, pydata-google-auth shouldn't fail at import time, so we should definitely fix that first.

[tswast]

I've fixed the issue and released https://pypi.org/project/pydata-google-auth/0.1.2/ in pydata-google-auth.