DatafileManager: HTTP errors are not logged

[fmck3516]

What I wanted to do

Check JavaScript SDK logging when the datafile request fails due to an HTTP error.

What I expected to happen

Error log being written (that's what the Java SDK does in case of HTTP errors).

What actually happened

No error log was written.

Steps to reproduce

Link to repository that can reproduce the issue: https://github.com/fmck3516/optimizely-js-incomplete-error-logging

Proposal to fix that: #783

[Tamara-Barum]

FSSDK-9615 to track internally

[raju-opti]

#904 handles this.