Add better error checking around 404s and actual database errors

[wildefires]

Description

This change does an explicit check for a RecordNotFound error and returns a 404 only in that case; returning 500 in other cases

Motivation and Context

Fixes #317

Currently, in the case that the database is removed from behind a running instance of flagr (network connection, password change, etc) the application returns a 404 to a GET request on localhost:13480/api/v1/flags/1000

How Has This Been Tested?

Tested locally by starting up an instance of flagr, verifying functionality, removing the backing database and then attempting to get the same flag again.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I don't believe there are tests covering this particular kind of failure case in crud_tests.go but I might have missed them - please do point it out and I'll add them if so

[codecov-io]

Codecov Report

Merging #318 into master will decrease coverage by 0.15%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage   81.88%   81.72%   -0.16%     
==========================================
  Files          26       26              
  Lines        1535     1538       +3     
==========================================
  Hits         1257     1257              
- Misses        209      211       +2     
- Partials       69       70       +1

Impacted Files	Coverage Δ
pkg/handler/crud.go	89.68% <50%> (-0.78%)	⬇️

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[wildefires]

Do you have any advice on writing a test case for this? Is it possible to remove access to the database during the test runs?

[zhouzhuojie]

Do you have any advice on writing a test case for this? Is it possible to remove access to the database during the test runs?

I think similar to https://github.com/checkr/flagr/blob/master/pkg/handler/crud_test.go#L145, you can have a subtest that stub again for getDB(). Not looking close enough though, stub and mock gorm may be tricky.

[zhouzhuojie]

LGTM, I will merge this and I can add a simple test in the following PR #337