Remove msg argument in assertError* functions and rename assertErrorMessage to

assertThrows;

Note the WPT testharness.js' assert_throw can't be used, because it needs to be
fed a String equal to the Error's constructor type name, which is mapped from a
list of "well-known errors" hardcoded in testharness.js; WebAssembly errors
don't appear in there.

Author: bnjbvr