Fix for doc build when Glymur is present but OpenJPEG is not

Read the Docs is unhappy because it installs Glymur, but not OpenJPEG.  This issue was noted and attempted to be fixed in PR #1180, but the exception that Glymur throws is actually `IOError`, not `ImportError`.

Incidentally, the change in Glymur of throwing an exception instead of a warning was introduced just a month ago with with this commit (https://github.com/quintusdias/glymur/commit/4ac5b575d20c2ccdd8b0901d8cb1d2a0442a70e6) for Glymur 0.7.0.