Better error handling

[nhz2]

Fixes #32 Fixes #36

This fixes #36 by porting the fix in JuliaIO/CodecZlib.jl#62 to this package.

The descriptions of the errors are from https://sourceware.org/bzip2/manual/manual.html#err-handling

The bzip2 manual doesn't explicitly describe how to check for truncated streams to fix #32 so I am using the method used in BZ2_bzRead in bzlib.c line 1204 from bzip2 v1.0.8:

      if (ret == BZ_OK && myfeof(bzf->handle) && 
          bzf->strm.avail_in == 0 && bzf->strm.avail_out > 0)
         { BZ_SETERR(BZ_UNEXPECTED_EOF); return 0; };

Here the input size being zero is used by Transcodingstreams to signal the end of the input stream.

    if code == BZ_OK
        if iszero(input.size) && !iszero(stream.avail_out)
            error[] = BZ2Error(BZ_UNEXPECTED_EOF)
            return Δin, Δout, :error
        else
            return Δin, Δout, :ok
        end

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 23 lines in your changes missing coverage. Please review.

Project coverage is 82.63%. Comparing base (5ce971b) to head (4866eeb).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/compression.jl	50.00%	11 Missing ⚠️
src/decompression.jl	60.71%	11 Missing ⚠️
src/libbzip2.jl	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   84.92%   82.63%   -2.29%     
==========================================
  Files           4        4              
  Lines         126      167      +41     
==========================================
+ Hits          107      138      +31     
- Misses         19       29      +10     

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