Bugfixes: v5.1 has missing unit test cases and error code coverage

[ppratikcr7]

Version where bug was found:
e.g "5.1.4"

Describe the bug
Spawned during testing #1425

• We are missing unit test cases for Segment Controller, we need to add that.
• We are also missing various non-happy paths error codes coverage, still we need to verify if all the swagger error codes are relevant or not. Below are the list of error codes missing in unit test cases as per swagger document: https://docs.google.com/spreadsheets/d/1hER9O-4fqguM_e8Ls7LzHCKevJKbGSM7GfTR0d0EvxI/edit?usp=sharing
• We also need to add test cases for 400 errors due to validators.
• The export experiment api unit test case is hitting post request for import api instead of get request for export.
• Swagger details are missing for various new apis.
• Analytics Controller tests has post request details for export csv data.
• Analytics Controller Swagger and Email send bugs and improvements #1733
• ExperimentClientController error code improvement and swagger updates #1736

Expected behavior
All these admin endpoint tests should be properly covered.

Screenshots

[VivekFitkariwala]

@amurphy-cl This is an improvement in the unit test cases for the backend we should add this in the next release. This will take some time for implementation and testing.