Discourage usage of NotImplementedError

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Proposal

Discourage usage of NotImplementedError.

Rationale

1. Its actual meaning is different than the name implies – according to RubyDoc it should be raised when a feature is not implemented on the current platform, think of low-level things like calling fork or fsync.
2. It's a descendant of ScriptError so a rescue block will not capture this. This might sound like a good thing but I believe we are using the mechanism in a way that it was not meant to be used.

Solutions

1. Create a RuboCop rule that either:

• Asks NoMethodError with a message whenever NotImplementedError is raised
• Create our own exception class for this and ask developers to raise that instead

2. Perhaps create an issue on Ruby Issue Tracking System so maybe we'll get a new exception class in the language itself

/cc @gitlab-org/maintainers/rails-backend