feat: implement js,wasm DataChannel OnError & OnClosing

[WofWca]

Description

• feat: implement js,wasm DataChannel.OnError
• feat: implement js,wasm DataChannel.OnClosing

FYI OnClosing is not implemented in the non-JS version of Pion,
so this is a JS-only method. Please consider if this is a good idea.

This is a follow-up to #479.

FYI I have not tested the OnError event (in particular obtaining errorMessage), but the both methods are largely copy-pasted from similar ones.

Reference issue

Partially addresses #519 (comment).

---

If I may, @albrow thank you so much for the initial implementation!
It's a bliss that it "just works"!

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.

Project coverage is 78.34%. Comparing base (59c7270) to head (e0a45f0).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
datachannel_js.go	0.00%	31 Missing and 2 partials ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3030      +/-   ##
==========================================
- Coverage   78.54%   78.34%   -0.20%     
==========================================
  Files          89       89              
  Lines       11078    11111      +33     
==========================================
+ Hits         8701     8705       +4     
- Misses       1890     1917      +27     
- Partials      487      489       +2     

Flag	Coverage Δ
go	80.12% <ø> (+0.03%)	⬆️
wasm	63.39% <0.00%> (-0.52%)	⬇️

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[Sean-Der]

Thank you @WofWca!

I went to write some tests, but wrtc supports neither :(