Annotate a few exceptions.

[cpovirk]

A couple notes:

• UnsupportedEncodingException: I didn't look much to see how likely a null argument was. But given that the class provides a no-arg constructor, which results in a null message, I saw no particular reason to restrict callers from passing a null message to the one-arg constructor, especially when most exception types support that.

• MatchException: The docs are clearer about allowing a null cause, but javac does generate calls that pass null for both the cause and the message.

• I ignored CoderMalfunctionError because I was uncertain whether to give it a nullable cause or not.

  • I generally err on the side of nullable, as in UnsupportedEncodingException above.

  • I see one test in Google's codebase that passes null, but doesn't care what it passes, and it could easily be changed not to pass somethign else.

  • I see a test in some version of Apache Harmony that passes null and verifies that it works.

  • But "clearly" it's "supposed" to have a non-null cause, similar to a type like InvocationTargetException or (possibly to a lesser extent, sadly) ExecutionException. For discussion of those types, see Consider annotating InvocationTargetException, ExecutionException, and CompletionException causes as non-nullable jspecify#490.

(prompted by google/xplat@7ab1a2c)