Gem::Package::TarReader::Entry now raises EOFError or returns nil
appropriately based on Ruby core IO.read and IO.readpartial behavior.

Zlib will respond accordingly by raising Zlib::GzipFile::Error on EOF.

When verifying a gem or extracting contents, raise FormatError similar
to other cases of corrupt gems.

Addresses a bug where Gem::Package would attempt to call size on nil
instead of raising a more descriptive and useful error, leading users
to assume the problem is internal to rubygems.

Remove unused error class TarReader::UnexpectedEOF that was never raised
since the NoMethodError on nil would happen first. Use EOFError instead.

https://github.com/rubygems/rubygems/commit/dc6129644b