Improve timeout handling

[FrederikBolding]

This PR improves timeout handling in the SnapController and in the execution services, it makes a handful of changes:

• Adds a timeout of 2 seconds to the ping request, sent to the execution environment after boot. This lets us fail more immediately if the execution environment has loaded but is non-responsive (e.g. if the execution env is down, the iframe loads a Chrome failure page, which still counts as the iframe loading).
• Adds maxInitTime as a constructor argument allowing for better configuration of how long Snaps (and the execution environment) has in total to boot before timing out. Previously maxRequestTime was used for the initialization timeout.
• Changes all of the error messaging around timeouts in an attempt to give more information than the previous vague error message: "The request timed out.".

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.60%. Comparing base (4d65e1d) to head (1981693).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2242   +/-   ##
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  Coverage   96.60%   96.60%           
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  Files         337      337           
  Lines        7602     7608    +6     
  Branches     1177     1180    +3     
=======================================
+ Hits         7344     7350    +6     
  Misses        258      258           

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