Add IOError for missing frames
Add IOError for missing frames

For some reason, I'm getting different errors for this in
different environments.

dwhswenson/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,openpathsampling/openpathsampling,dwhswenson/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,choderalab/openpathsampling,dwhswenson/openpathsampling,openpathsampling/openpathsampling,dwhswenson/openpathsampling