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.

test-utils/domMock.js

@@ -304,6 +304,13 @@ module.exports = function(options) {
 					parentNode: null,
 					childNodes: [],
 					attributes: {},
+					contains: function(child) {
+						while (child != null) {
+							if (child === this) return true
+							child = child.parentNode
+						}
+						return false
+					},
 					get firstChild() {
 						return this.childNodes[0] || null
 					},