return correct error on duplicate email while updating user

[tinfoil-knight]

Closes: #7249

Currently, a user updating their email to one that's already in use returns an 500 Internal Server Error. This PR updates the implementation to ensure that a 400 Bad Request error with a description is returned instead.

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

• Create a user (using the POST /users endpoint)
• Attempt to modify the user (using the PUT /user/current endpoint)
• Verify that you're receiving an error with HTTP status code 400 with a message mentioning that the email already exists

If this is a bugfix, which Traffic Control versions contained the bug?

Wasn't able to track down the exact release where this bug originated. This bug is present in master branch & #3996 is the PR where the endpoint was first written for API V3.

PR submission checklist

• This PR has tests
• This PR has documentation
  PR doesn't need any separate documentation since only the error handling behaviour has been changed. API docs only document the request & response structure.
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[tinfoil-knight]

Wanted to confirm if a test needs to be added here.

I was looking at the test files & I'm slightly confused as to how/where users are added in the mock database. (I'd need to add 2 users & then attempt to change the email for 1 of the users to an email already in use by the other for a functional test)

One of the relevant file is here: https://github.com/apache/trafficcontrol/blob/master/traffic_ops/testing/api/v5/user_current_test.go

[codecov]

Codecov Report

Merging #7394 (29495cb) into master (34a601f) will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #7394   +/-   ##
=========================================
  Coverage     26.12%   26.12%           
  Complexity       98       98           
=========================================
  Files           648      648           
  Lines         76280    76280           
  Branches         90       90           
=========================================
  Hits          19925    19925           
  Misses        54520    54520           
  Partials       1835     1835           

Flag	Coverage Δ
golib_unit	52.47% <ø> (ø)
grove_unit	4.60% <ø> (ø)
t3c_unit	5.33% <ø> (ø)
traffic_monitor_unit	20.43% <ø> (ø)
traffic_ops_integration	69.34% <ø> (ø)
traffic_ops_unit	19.70% <0.00%> (ø)
traffic_stats_unit	10.14% <ø> (ø)
unit_tests	22.55% <0.00%> (ø)
v3	57.79% <ø> (ø)
v4	79.09% <ø> (ø)
v5	78.53% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
traffic_ops/traffic_ops_golang/user/current.go	0.00% <0.00%> (ø)

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[ocket8888]

Looks good, works, fixes the bug