`GithubCacheEntry` is unique by `[:repository_id, :scope, :key,
:version]`. We check for its existence and return a smoother error
message when the cache entry already exists.
If a customer tries to create a cache entry that already exists at the
same time, a race condition could cause two threads to attempt to create
the same cache entry, resulting in an HTTP 500 error. We see a few
examples of this every day.
We already have a check and a test for the already existing case. But
it's not very robust to catch the database level exception.
`Sequel::ValidationFailed` exception is raised when unique validation
fails at the Sequel model level. `Sequel::UniqueConstraintViolation`
exception is raised when uniqueness validation passes at the Sequel
model level but fails at the database level due to concurrency.
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