Refactor API handler wrapping

[ocket8888]

This refactors api.Handler such that the return type is a single error instead of a user-facing error, a system-facing error, and a status code. It then handles the error specially if it is an api.Errors implementation. I also updated usage accordingly throughout the two endpoint handling packages that use api.Wrap.

Essentially this reduces

code, userErr, sysErr := someCall(inf)
if userErr != nil || sysErr != nil {
    return code, userErr, sysErr
}

to

err := someCall(inf)
if err != nil {
    return err
}

and

err := tx.QueryRow("My query", args...)
if err != nil {
    return http.StatusInternalServerError, nil, err
}

to

err := tx.QueryRow("My query", args...)
if err != nil {
    return err
}

---

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

This is just a refactor; behavior should be unchanged and the existing tests should continue to pass.

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR doesn't need a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY

[codecov]

Codecov Report

Attention: 267 lines in your changes are missing coverage. Please review.

Comparison is base (3d933d0) 29.09% compared to head (870c26a) 32.06%.

Files	Patch %	Lines
traffic_ops/traffic_ops_golang/server/servers.go	7.95%	240 Missing and 3 partials ⚠️
...traffic_ops_golang/cdnfederation/cdnfederations.go	17.24%	24 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7926      +/-   ##
============================================
+ Coverage     29.09%   32.06%   +2.97%     
  Complexity       98       98              
============================================
  Files           605      722     +117     
  Lines         77535    82925    +5390     
  Branches         90      970     +880     
============================================
+ Hits          22558    26590    +4032     
- Misses        52894    54183    +1289     
- Partials       2083     2152      +69     

Flag	Coverage Δ
golib_unit	53.85% <ø> (+<0.01%)	⬆️
grove_unit	12.02% <ø> (ø)
t3c_unit	5.88% <ø> (ø)
traffic_monitor_unit	25.47% <ø> (ø)
traffic_ops_unit	22.15% <19.81%> (+0.06%)	⬆️
traffic_portal_v2	74.37% <ø> (?)
traffic_stats_unit	10.78% <ø> (ø)
unit_tests	29.41% <19.81%> (+3.46%)	⬆️

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