Add ZlibError type and move initialize into startproc

[nhz2]

This PR applies parts of JuliaIO/CodecBzip2.jl#41 and JuliaIO/CodecZstd.jl#74 to this package.

This PR turns the initialize function into a noop. The startproc function now automatically handles the initialization if the stream state is null.

Fixes #89 fixes #59 and partially fixes #48 (a future PR is required to prevent memory leaks)

For example, decompressing a truncated stream:

julia> using CodecZlib

julia> transcode(ZlibDecompressor,UInt8[])

master:

ERROR: zlib error: <no message> (code: -5)

This PR:

ERROR: ZlibError: the compressed stream may be truncated

[codecov]

Codecov Report

Attention: Patch coverage is 57.37705% with 26 lines in your changes missing coverage. Please review.

Project coverage is 82.97%. Comparing base (98ccccd) to head (84de9cd).

Files with missing lines	Patch %	Lines
src/compression.jl	47.82%	12 Missing ⚠️
src/decompression.jl	60.00%	12 Missing ⚠️
src/libz.jl	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   91.66%   82.97%   -8.69%     
==========================================
  Files           4        4              
  Lines         156      188      +32     
==========================================
+ Hits          143      156      +13     
- Misses         13       32      +19     

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