MIMParser: put get_source_id_for_source_name() error code in a constant
MIMParser: put get_source_id_for_source_name() error code in a constant

Instead of repeating -1, use a descriptively named constant. This constant
should be defined in BaseParser but for the time being let us not touch
that module.

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