The error_pc was only used to calculated the relative error offset.
Switching to an error_offset directly will allow us later to give the
decoder a base offset. Thereby we can get correct error
positions even when the decoder is executed on multiple memory chunks,
which will happen with streaming compilation. With this change I also
had to provide "kind of" reasonable error position in the
StreamingDecoder.

R=clemensh@chromium.org

Change-Id: I736fa082c51c64334d23771061acf97e2c47778e
Reviewed-on: https://chromium-review.googlesource.com/502909
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45264}