encoding/base32: ensure base32 decoder propagates errors correctly

A number of issues in decoder.Read and newlineFilteringReader.Read were
preventing errors from the reader supplying the encoded data from being
propagated to the caller.  Fixing these issues revealed some additional
problems in which valid decoded data was not always returned to the user
when errors were actually propagated.

This commit fixes both the error propagation and the lost decoded data
problems.  It also adds some new unit tests to ensure errors are handled
correctly by decoder.Read.  The new unit tests increase the test coverage
of this package from 96.2% to 97.9%.

Fixes #20044

Change-Id: I1a8632da20135906e2d191c2a8825b10e7ecc4c5
Reviewed-on: https://go-review.googlesource.com/42094


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>