MIMParser: croak upon failure to retrieve MIM source IDs
MIMParser: croak upon failure to retrieve MIM source IDs

Without this, if the relevant metadata is not present in the xref
database we are working with the parser will make a lot of noise but only
actually fail upon trying to add a new xref to the database, i.e. much, much
later. Let us fail when the problem actually occurs, shall we?

Use croak() rather than 'return 1' because this is a set-up error rather
than a data-processing one.

Ensembl/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl