@tus/server: Metadata on namingFunction + propagate error

[fenos]

This small PR simply allows error propagation in the namingFunction.

Currently, if an error is thrown from the namingFunction the error is swallowed and a 500 error code is returned.
This behavior is not ideal, in my use case I rely on the metadata in order to create a valid uploadID.

For this reason the metadata is validated in the naming function. If some of the fields don't pass the validation instead of returning a 400 status code, the application returns a non-ideal 500

[fenos]

@Murderlon i think it makes a lot of sense to also pass the parsed Metadata in the namingFunction as an optional second param for example:

function namingFunction(req, metadata) {
  if (!metadata) {
     throw new Error('must provide metadata')
  }
  
  // validate metadata
  return metadata.fileName
}

[Murderlon]

Sounds good 👍

We can add the metadata argument in this PR as well

[fenos]

Done 🎉

[Murderlon]

I'll do a release tomorrow.