remove LibraryNotFoundError, use RuntimeError instead, closes #274, #278

LibraryNotFoundError isn't needed because we already error out during
the import of glymur if neither openjpeg nor openjp2 can be found.

When read_bands is used and the library version is not at least 2.0.0,
use RuntimeError instead of LibraryNotFoundError.
merge branch 'issue278' into devel