fix: fetch: handle invalid server responses, set responseType

[aarongranick-okta]

• Catch error from json parsing caused by invalid/malformed server response and return a well structured error rather than the raw TypeError. SIW has unit tests for these scenarios. Adding tests here to cover the same functionality.
• Always return responseText as a string. SIW had unit tests which revealed this quirk/bug in this library. If response type is JSON, the object will also be returned as responseJSON and responseType will be set to JSON.

[swiftone]

This feels weird. It only makes sense when I scroll down and notice this is all defining respHandler, which is passed either the raw text, an object from JSON, or an error object.

isJSON is actually the OPPOSITE - it's not JSON, it's an object. When isJSON is true, this calls JSON.stringify() (converting the object to JSON).

I suggest:

1. Defining respHandler outside of fetchRequest - it distracts
2. Changing isJSON to something else (isObj, or perhaps notYetJSON)

[aarongranick-okta]

Good feedback! I have refactored for better readability. Please take another look

[bretterer]

Looks good to me with a small nit

[bretterer]

nit: we should avoid using console.error as much as possible. console is not 100% reliable and can break items based on chrome and other browser extensions. example is when firebug removes native console commands and replaces them with their own, which caused issues in the past.