Better error messages when jars are unavailable

[ahnlabb]

When the build process has failed and bioformats_package.jar is unavailable (e.g. download failure) the user gets cryptic error messages when calling functions in the package (#5). The risk of this happening may increase if we need to download more assets (e.g. #8). It would be better to warn the user that these files are missing when they import the package and give more informative error messages.