Rework lots of error references

[MikeBishop]

Fixes #4352.

Expands on error handling discussion and clarifies difference from HTTP status codes which indicate error response. Also, some editorial consistency fixes:

• The references to errors seem to have missed our switch to use "; see XXXX." where possible.
• Somewhere along the way, we stopped referring to the error handling section when defining errors.

I've done a sweep and believe these are consistent now.

@theri, your review would be appreciated.

[renghardt]

Thank you, this is already helpful. I would like the text to be yet a bit more explicit, see my suggestions.

Also, could you add another reference to {{errors}} where you first mention error codes, i.e., after "error code H3_REQUEST_INCOMPLETE" in Section 4.1 (at the bottom of page 12 in -32), please?

[renghardt]

Looks good to me, thanks!

[janaiyengar]

one nit

[LPardue]

lgtm thank mike