use integer for exon keys, fixes #66
improve filter-interpretable-exons error handling, fixes #49, fixes #50
Merge pull request #68 from mheuer-nmdp/error-reporting

Improve filter-interpretable-exons error handling, fixes #49, fixes #50
Merge pull request #67 from mheuer-nmdp/int-keys

Use integer for exon keys, fixes #66