blocks referencing stage weren't getting imported correctly in production code, preventing projects
that used them from loading at all (BSOD)
resolves#31
This fixes an issue where the md5 in the sb2 json was not matching the md5 from the assets server.
This fix avoids encoding the sound data as a wav twice.
project validation was failing when trying to save a project originally from 1.4 as an sb3 because
the md5s in the produced sb2 file were not of the right format
Add ValidationError to help distinguish between unrecoverable errors
while decoding an `.sb` file and encoding to Scratch 2 data.
Export AssertionError and ValidationError so users of the converter can
know when an error is from validating the Scratch 1 file (as in it
likely is not an Scratch 1 file) or from making sure the Scratch 1 file
data matches expected ranges (like not making memory buffers that are
too big).
