Make errors and their messages more accurate and helpful (#2536)

Also, I normalized them to all be sentences for consistency, and I moved
the reentrancy check from `m.mount` to `m.render` to be a little more
helpful. The router change during mounting is inconsequential and only
to avoid the new modified error, and the change to the update loop is to
send the original error if an error occurred while initializing the
default route. (This is all around more useful anyways.)

And while I was at it, I fixed an obscure bug with sync redraws.

docs/change-log.md

@@ -17,6 +17,14 @@
 
 ### Upcoming...
 
+*Note for later: release as semver-minor.*
+
+- Improved error messages in multiple places. ([#2536](https://github.com/MithrilJS/mithril.js/pull/2536) [@isiahmeadows](https://github.com/isiahmeadows))
+- The redraw reentrancy check was moved from `m.mount` to `m.render` and its error message was updated accordingly. ([#2536](https://github.com/MithrilJS/mithril.js/pull/2536) [@isiahmeadows](https://github.com/isiahmeadows))
+    - This is unlikely to break people because if you were to do it with `m.render` directly before now, you'd corrupt Mithril's internal representation and internal errors could occur as a result. Now, it just warns you.
+- For a better debugging experience with `m.route` route resolvers, errors on `onmatch` in the default route are left unhandled and errors in `onmatch` in other routes are logged to the console before redirecting. ([#2536](https://github.com/MithrilJS/mithril.js/pull/2536) [@isiahmeadows](https://github.com/isiahmeadows))
+- Bug fix with `m.redraw` where if you removed a root that was previously visited in the current redraw pass, it would lose its place and skip the next root.
+
 -->
 
 ### v2.0.4