Improves the error handling for the swf crate:
* Custom swf::error::Error type added to handle various errors
in SWF parsing.
* Invalid parsing of tags/AVM1 ops results in a Error::ParseError
that can include info about the underlying failure.
* Implement Display for these errors. Output descriptive
names for the tag/opcode when it fails to parse.
* Handle out of bounds reads in avm1::Reader::read_slice.
Previously this would panic, now it returns an io::Error.
Closes#85.
Mike Welsh