Is your feature request related to a problem? Please describe.

MikroOrm currently uses ValidationError as a catch-all for many different types of errors. The same ValidationError is thrown when an entity is not found (from findOrFail) or if an entity is referenced that is not registered.

I would argue that these are very different types of errors. Referencing an entity that isn't managed is a fatal error that represents a fundamental issue with the application. Not finding an entity in the database is a runtime error that may be expected if using user input to retrieve the entity.

Using the same class for all of these errors, with the only difference being the message string, makes it difficult to write error handling code. For example, I may want to catch "NotFound" errors and provide a friendly message to the user. But something like "wrongPropertyType" is a bug and I'd want to log it and give the user a generic "SERVER ERROR" message as to not leak details about the application via the API.

With the current error structure I can't use typical strategies like checking the instanceof an error and handling it accordingly. The best I can do is hacky fuzzy string matches on the error message string.

Describe the solution you'd like
My preference would be to use specific error classes for specific errors. ValidationError has lots of static factory methods for various error types. That indicates to me that you could turn each of those static methods into subclasses specific to that error.

I'd also differentiate between runtime errors like "NotFound" and more fatal errors like problems like mismatching schemas/types or entities not being registered.

Describe alternatives you've considered
If for some reason there is resistance to having more specialized error classes can we at least have a specific code set on the ValidationError to make it easier to determine what type of error it is? This would be less ideal than using subclasses but at least it would allow me to handle errors in a more robust way than checking that the message contains "not found".

Additional context
For my specific use-case: I'm using MikroORM to drive a GraphQL API. I'd like to use findOneOrFail when retrieving entities from user input. If someone provides an ID that doesn't exist that's not a bug in my application, it's a user error and should be handled as such. But right now I can't easily differentiate between a "NotFound" error or a bug like I changed a property name and forgot to update my retrieval code.

I could forgo findOneOrFail and just use findOne, guard against null values, and throw my own error... but that's a lot of boilerplate code.