Don't log update checker errors

They're annoying when there's no internet connection and unlikely to be useful
to anyone.
Merge pull request #1154 from realm/tg-no-log-error

Don't log update checker errors
fix comment typo
cancel transaction on RLMRealm dealloc if still in write transaction
improve dealloc rollback log message
fixed log message wrapping
fix error message when no migration block is specified for old Realms
changelog
Merge pull request #1167 from realm/al-version

Fix error message when no migration block is specified for old Realms
Merge pull request #1161 from realm/jp-auto-cancel

cancel transaction on RLMRealm dealloc if still in write transaction
Add -[RLMRealm endReadTransaction]

Also leaves newly created RLMRealm instances without a read transaction as
this makes it simple to do so.

Things to bikeshed:

1. The name `endReadTransaction`. I'm not a fan. `close` is wrong since it
   doesn't actually close the file and anything involving `release` sounds
   refcounting related.

2. What should `refresh` return when called on a Realm with no read
   transaction? Currently it always returns `false`. Maybe it should always
   return `true`?

3. Do notifications make any sense for realms with no read transaction?
   Currently they work as usual (except with no notification collapsing because
   `has_changed()` always returns true when the shared group doesn't have a
   read lock).
Rename endReadTransaction to invalidate