Fixed compilation issues

[TMSCH]

1. TS 2.4 String enums compile in a format that makes it incompatible with Closure Compiler.

enum StrEnum { value = 'value'; otherValue = 'otherValue' }

Usage: StrEnum.value

Will compile in:

StrEnum = { 'value': 'value', 'otherValue': 'otherValue' }

StrEnum.value;

Notice quoted properties. This breaks obfuscation.

2. Error subclassing does not extend the prototype, so the toExposedError method wasn't available. https://github.com/Microsoft/TypeScript/issues/13965

[codecov-io]

Codecov Report

Merging #42 into master will decrease coverage by <.01%. The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   87.27%   87.26%   -0.01%     
==========================================
  Files          34       34              
  Lines        1430     1429       -1     
  Branches      168      168              
==========================================
- Hits         1248     1247       -1     
  Misses        182      182

Impacted Files	Coverage Δ
ts/protocol/data.ts	100% <100%> (ø)	:arrow_up:
ts/protocol/preload_request.ts	100% <100%> (ø)	:arrow_up:
ts/protocol/rpc_messages.ts	95.38% <100%> (+0.07%)	:arrow_up:
ts/spi/provider_frame.ts	85.93% <100%> (+0.05%)	:arrow_up:
ts/api/api.ts	69.68% <100%> (-0.17%)	:arrow_down:
ts/protocol/errors.ts	89.02% <100%> (+0.13%)	:arrow_up:
ts/protocol/post_messages.ts	100% <100%> (ø)	:arrow_up:
ts/protocol/utils.ts	82.53% <88.88%> (+0.13%)	:arrow_up:
ts/api/provider_frame_elem.ts	61.9% <0%> (-0.89%)	:arrow_down:
... and 1 more

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