fix(agent): Suppress creation of err logs for req failures

[balamurali27]

This is possible by setting http_status_code to < 500. Using custom
status code 420 for this purpose.

• Need to figure out way to push whole traceback to sentry before doing this. Seems neither press nor agent does this at this point.
  • Nicely formatting the trace (frappe/agent@3ac5721) also doesn't seem to help

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 38.29%. Comparing base (62f834a) to head (4ef2a80).

Files with missing lines	Patch %	Lines
press/exceptions.py	60.00%	2 Missing ⚠️
press/agent.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2433   +/-   ##
=======================================
  Coverage   38.28%   38.29%           
=======================================
  Files         378      378           
  Lines       29771    29777    +6     
=======================================
+ Hits        11399    11403    +4     
- Misses      18372    18374    +2     

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