Truncate error class, error message and context

[nickdowell]

Goal

Reduce number of oversized payloads due to exceeding back-end's size limit.

Follow-up to #1449

Changeset

Truncates additional strings that can potentially get long:

• context
• errorClass
• errorMessage

Testing

[github-actions]

Bugsnag.framework binary size did not change - 706,120 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.2%    +480  +0.2%    +480    __TEXT,__text
  -1.9%    -480  -1.9%    -480    [__TEXT]
  [ = ]       0  [ = ]       0    TOTAL

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