retry on all thrown fetch errors

[jdelbick]

We used to retry on all thrown http errors:

node-fetch-retry/index.js

Line 29 in 4161e43

(error !== null || (retryOptions.retryOnHttpResponse && (retryOptions.retryOnHttpResponse(response))))

Then when we refactored the code, the behavior changed and only retried on error.code 5xx which brought in this regression: #60
To be more thorough, we added a customizable function called retryOnHttpError and changed the default to retry on all FetchErrors: #63

The goal now is to expand the behavior (in addition to this change: #87) to call retryOnHttpError for all thrown errors, not just FetchError. This will make it easier to implement to make node-fetch-retry pluggable: #86

Developers can still use the custom retryOnHttpError function to add special behavior, like only retrying on FetchError's.

[alexkli]

Note that #87 should be done first so we don't blindly retry on developer errors in the callbacks.