Fix faulty assertion of IsValidTimeZoneName

In ParseTemporalTimeZoneString, a time zone name may meet the grammar for
time zone names while still not being a supported time zone name.
Therefore, this should be a throw rather than an assertion. This was
probably left over from when the grammar was defined more loosely as "any
string that is a supported time zone name".

spec/abstractops.html

@@ -1466,7 +1466,7 @@ <h1>ParseTemporalTimeZoneString ( _isoString_ )</h1>
         1. Let _offsetNanoseconds_ be _sign_ × (((_hours_ × 60 + _minutes_) × 60 + _seconds_) × 10<sup>9</sup> + _nanoseconds_).
         1. Let _offsetString_ be ! FormatTimeZoneOffsetString(_offsetNanoseconds_).
       1. If _name_ is not *undefined*, then
-        1. Assert: ! IsValidTimeZoneName(_name_) is *true*.
+        1. If ! IsValidTimeZoneName(_name_) is *false*, throw a *RangeError* exception.
         1. Set _name_ to ! CanonicalizeTimeZoneName(_name_).
       1. Return the new Record: {
         [[Z]]: *undefined*,