Bug 575641 - restore silence on error in readDocument()

That produces lot of irrelevant log entries with async editor diff
operations on quickly changing resources.

Change-Id: I7eee3163df092256f367c48400df453525e6f868
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.text/+/185357
Tested-by: Platform Bot <platform-bot@eclipse.org>