Don't panic when gl.create_buffer() fails on webgl

[Dinnerbone]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
n/a

Description
In webgl gl.create_buffer() is fallible, and can return null (or Err, thanks to glow) when the device is unable to allocate any more buffers. We unwrap and crash instead of treating it as we do other OOM cases.

Side note: Has there been a discussion already about potentially disallowing unwrap() going forwards? So many unwraps are quite fragile :( expect forces developers to give a reason to why the panic should never happen.

Testing
Passes existing tests.

[codecov-commenter]

Codecov Report

Merging #3396 (ca2b2b6) into master (0849e78) will increase coverage by 0.26%.
The diff coverage is 100.00%.

❗ Current head ca2b2b6 differs from pull request most recent head 0cd527c. Consider uploading reports for the commit 0cd527c to get more accurate results

@@            Coverage Diff             @@
##           master    #3396      +/-   ##
==========================================
+ Coverage   65.23%   65.49%   +0.26%     
==========================================
  Files          86       66      -20     
  Lines       42772    37392    -5380     
==========================================
- Hits        27901    24490    -3411     
+ Misses      14871    12902    -1969     

Impacted Files	Coverage Δ
wgpu-hal/src/gles/device.rs	81.40% <100.00%> (ø)
wgpu-types/src/assertions.rs	0.00% <0.00%> (-37.50%)	⬇️
wgpu-hal/src/auxil/renderdoc.rs	40.50% <0.00%> (-3.55%)	⬇️
wgpu-hal/src/lib.rs	24.03% <0.00%> (-2.89%)	⬇️
wgpu-types/src/lib.rs	86.53% <0.00%> (-0.92%)	⬇️
wgpu-hal/src/gles/egl.rs	38.00% <0.00%> (-0.50%)	⬇️
wgpu-core/src/device/mod.rs	66.44% <0.00%> (-0.29%)	⬇️
wgpu-hal/src/vulkan/instance.rs	38.18% <0.00%> (-0.19%)	⬇️
wgpu/src/backend/direct.rs	56.90% <0.00%> (-0.09%)	⬇️
wgpu-core/src/command/render.rs	62.71% <0.00%> (-0.06%)	⬇️
... and 23 more

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[cwfitzgerald]

Side note: Has there been a discussion already about potentially disallowing unwrap() going forwards? So many unwraps are quite fragile :( expect forces developers to give a reason to why the panic should never happen.

We haven't - my gut feeling is that the verbosity of expect might get very annoying for trivial stuff.