guard more of the filesystem manipulations with
handlers and log errors instead of letting them
be raised and show up as internal errors
related to PR 13403
original commit: 94b54db11823b4683234424174aca2836d422204
Changes the implementation of highlight-range so that it
only recomputes all of the new locations from the positions
when on-reflow is called (otherwise only computing the
relevant ones) and make the on-reflow callback chop itself
up, in case there are lots of highlighted ranges to avoid
tying up the event loop.
Changes searching so that it doesn't neccessarily compute
the entire search results in a single event callback
(but also make it start the computation more aggressively)
Overall, this changes the strategy from one that, for any potentially
long-running callback, just tried to push it off into the future, into
a strategy that tries to avoid long-running callbacks by breaking the
work up into chunks, but starting the first chunk immediately (in a
low-priority callback).
Also, misc other changes to make this work better and generally clean
things up.
original commit: 5197649cb7556abd96282d3a838c4ba4f7c8de8c
