Error reporting should be better

Currently error handling is poor:

• failure to allocate memory results in error message sent to stderr and segfault (e.g. in roaring_bitmap_add())
• there is no way to distinguish between memory allocation errors and serialization format errors (portable_deserialize())

Both problems can be solved with integer error codes, e.g. 0 -- operation successful, -1 -- memory allocation error, -2 -- serialization format error, etc. Error codes should be returned for all operations which modify bitmaps.

There is, of course, an issue of atomicity. If error occurs inside some modification operation, bitmap can enter half-broken state. Atomicity guarantee would be too expensive from performance perspective. But we need to at least guarantee that bitmaps can be safely roaring_bitmap_free()d, even after an error. All other operations are UB.

Start by examining roaring_bitmap_add() and portable_deserialize(), then identify the other operations that modify bitmaps. Define integer error-code behavior for allocation and serialization failures, including the guarantee that roaring_bitmap_free() remains safe after an error. The issue explicitly permits non-atomic mutations, so document the resulting usage constraints.