#317 Cope better with multiple features with the same name

[Kim Rutherford]

When the loader queries for a feature by name and gets two results it throws an error like:

c29f51c4cc86096d: DBIx::Class::Schema::txn_do(): can't find feature using identifier: rga4 (error too many matches for rga4)

In this case the name matched a gene and an allele. It should cope. The error above was thrown soon after this line in ExtensionProcessor.pm:
if ($identifier =~ /^(PomBase|GeneDB_?Spombe):([\w\d.-]+)/i) {

I'm leaving the priority low for now as an error is throw if this happens. It's not the best error message but it's enough to point at the problem.