signature builder: don't raise

[drjova]

• Don't raise when the schema is unknown.

• Don't raise when schema don't match the value pattern.

• Ref: cern-sis/issues-inspire/issues/60

[drjova]

@MJedr which exception?

[MJedr]

SchemaUIDConflict and UnknownUIDSchema, but it's ok, we don't handle it anywhere

[drjova]

@michamos could you please double check if this fits what we discussed on Zulip?