Read a page as far as it decoded, and still refuse one that did not

[tannevaled]

reader v0.6.0 keeps what a broken filter chain did produce and says so, instead
of returning a fragment with a nil error. Taking it without deciding what this
package wants would have changed the contract by accident in both directions,
so the decision is made here and written down.

A page whose content decoded part of the way is now read that far. 263 streams
in 212 of the 1 633 real forms — 13.0% of them — cannot be decoded cleanly, and
half a page of text is worth more to somebody searching than none of it.

A page that decoded no bytes at all stays an error. That is not symmetry for
its own sake: go-pdfkit/latex turns a document into LaTeX, and a conversion
that silently produces an empty document from an unreadable page is worse than
one that says it could not read it. Its own test caught this the moment the
dependency moved, which is the test doing its job.

The pair of tests states both halves. A stream of 108 bytes holding 301 pieces
of text, cut to 80, gives 147 of them and no error; the same stream declared as
Flate and holding no Flate at all still gives an error.

Also on the fixes this brings: the /Crypt filter, which 209 files — 12.8% of
the real forms — carry and which used to make 209 of them decode to nothing
readable; a second map-order non-determinism, in indexObjectStreams; and files
whose crypt filters say /Identity, which need no password and now open.