fix(3225): organize the return values of _parseHook [2]

[y-oksaku]

Context

Currently, the _parseHook method returns Object or null or throw Error.
However, their return values are not properly conditioned.

Objective

Organize the return values by the following:

• return Object (parsed result): the scm is correct and Screwdriver do actions with the webhook (e.g. push event)
• return null: the scm is correct and Screwdriver do nothing (e.g. delete event)
• throw Error: the scm is not correct

Then we can handle whether the webhook is correct or not at here.

References

Issue

• Webhook returns 204 even when a parsing error occurs screwdriver#3225

PR

• fix(3225): handling when the webhook is incorrect [4] scm-router#37

License

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[tkyi]

Lgtm
Can you update the test title from false to true (line 2177)
https://github.com/screwdriver-cd/scm-gitlab/pull/61/files#diff-af95cb6f5f65fe70216ddce70325e785438d5aaff956a0666ebf46ef6402f526R2177

[y-oksaku]

Thank you! I fixed it.

[sd-buildbot]

🎉 This PR is included in version 4.0.1 🎉

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