Remove unnecessary nested Locker and Isolate::Scope use

[dylanahsmith]

cc @GustavoCaso

While reviewing Shopify#39, I noticed that ExceptionError used

  Locker locker(iso);
  Isolate::Scope isolate_scope(iso);

even though ExceptionError isn't exported to Go code and the caller should already have taken the lock and entered that isolate scope. That PR was trying to preserve this behaviour for existing code, but we should actually just remove those lines.

I've also made ExceptionError static, making it clearer that it isn't used outside that c file.

[codecov]

Codecov Report

Merging #275 (7a727da) into master (ede7cee) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #275   +/-   ##
=======================================
  Coverage   95.88%   95.88%           
=======================================
  Files          17       17           
  Lines         583      583           
=======================================
  Hits          559      559           
  Misses         15       15           
  Partials        9        9           

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[genevieve]

Looks good minus the formatting fix needed

[GustavoCaso]

That PR was trying to preserve this behaviour for existing code, but we should actually just remove those lines.

Fantastic. it would simply Shopify#39. Tomorrow I will apply the changes to the Heap snapshot PR