Editorial: make ISO-2022-JP encoder perform error state switch

This way you can invoke its handler directly without going the process algorithm as is needed to fix #235. This also avoids the need for "prepend" in the "process" algorithm.

Additionally, this commit adds two clarifying asserts to the "process" algorithm documenting what error modes can be in effect when.

Author: annevk

encoding.bs

@@ -303,6 +303,12 @@ happening.
  <var>encoderDecoderInstance</var> is a <a for=/>decoder</a> instance, otherwise
  "<code>fatal</code>".
 
+ <li><p>Assert: if <var>encoderDecoderInstance</var> is an <a for=/>encoder</a> instance,
+ <var>mode</var> is not "<code>replacement</code>".
+
+ <li><p>Assert: if <var>encoderDecoderInstance</var> is a <a for=/>decoder</a> instance,
+ <var>mode</var> is not "<code>html</code>".
+
  <li><p>Assert: if <var>encoderDecoderInstance</var> is an <a for=/>encoder</a> instance,
  <var>item</var> is not a <a>surrogate</a>.
 
@@ -340,9 +346,9 @@ happening.
    <dd><a>Push</a> U+FFFD to <var>output</var>.
 
    <dt>"<code>html</code>"
-   <dd><a>Prepend</a> U+0026, U+0023, followed by the
-   shortest sequence of <a>ASCII digits</a> representing <var>result</var>'s
-   <a>code point</a> in base ten, followed by U+003B to <var>input</var>.
+   <dd><a>Push</a> the result of <a>UTF-8 encoding</a> U+0026 (&amp;), U+0023 (#), followed by the
+   shortest sequence of <a>ASCII digits</a> representing <var>result</var>'s <a>code point</a> in
+   base ten, followed by U+003B (;) to <var>output</var>.
    <!-- &# ... ; -->
 
    <dt>"<code>fatal</code>"
@@ -2938,8 +2944,17 @@ and <var>code point</var>, runs these steps:
   <p class=note>If <var>pointer</var> is non-null, it is less than 8836 due to the nature of
   <a>index jis0208</a> and the <a>index pointer</a> operation.
 
- <li><p>If <var>pointer</var> is null, return <a>error</a> with
- <var>code point</var>.
+ <li>
+  <p>If <var>pointer</var> is null, then:
+
+  <ol>
+   <li><p>If <a>ISO-2022-JP encoder state</a> is <a lt="ISO-2022-JP encoder jis0208">jis0208</a>,
+   then <a>prepend</a> <var>code point</var> to <var>ioQueue</var>, set
+   <a>ISO-2022-JP encoder state</a> to <a lt="ISO-2022-JP encoder ASCII">ASCII</a>, and return three
+   bytes 0x1B 0x28 0x42.
+
+   <li><p>Return <a>error</a> with <var>code point</var>.
+  </ol>
 
  <li><p>If <a>ISO-2022-JP encoder state</a> is not
  <a lt="ISO-2022-JP encoder jis0208">jis0208</a>,