[TAP] Return a clear error message in case of duplicated items in UPLOADs.

Before correction, if two uploaded tables have been submitted by the user with
the same name, or if one uploaded table contained duplicated column names, an
obscure error message coming from the database was returned to the user.

Now, duplicated items (tables and columns) are searched before ingestion in the
database. When one is detected, an error is immediately returned to the user and
the query is aborted.