Replace defer w.Close() with explicit error checking

[robertodauria]

This PR replaces a defer w.Close() adding explicit error checking.

The documentation of Close() states that an object becomes available on GCS only after Close() has been successfully called.
Particularly, permission errors are only reported when closing the writer.

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1023

• 12 of 12 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 93.533%

---

Totals
Change from base Build 1016:	0.02%
Covered Lines:	1909
Relevant Lines:	2041

---

💛 - Coveralls

[stephen-soltesz]

Reviewed 4 of 4 files at r1.
Reviewable status: 0 of 1 LGTMs obtained