Add `humanize_error_code` generic case

[kaiserdan]

The humanize_error_code function should have a generic case for error codes that aren't explicitly defined. I sometimes get a 70007 error which then causes an error in the module. Happy to make a PR for this if you like.

(mint 1.6.0) lib/mint/http2.ex:823: Mint.HTTP2.stream/2
(mint 1.6.0) lib/mint/http2.ex:1397: Mint.HTTP2.maybe_concat_and_handle_new_data/2
(mint 1.6.0) Lib/mint/http2. ex:1402: Mint.HTTP2.handle_new_data/3
(mint 1.6.0) lib/mint/http2/frame.ex:99: Mint.HTTP2.Frame.decode_next/2
(mint 1.6.0) lib/mint/http2/frame.ex:237: Mint.HTTP2.Frame.decode_goaway/3
(mint 1.6.0) lib/mint/http2/frame.ex:472: Mint.HTTP2.Frame.humanize_error_code (70007)
** (FunctionClauseError) no function clause matching in Mint.HTTP2.Frame.humanize_error_code/1