Add warnings for obvious syntax errors in format strings.

Add warnings for obvious syntax errors in format strings.

* module/language/tree-il/analyze.scm (&syntax-error): New variable.
  (format-string-argument-count): Throw to &SYNTAX-ERROR when a syntax
  error in a format string is encountered.
  (format-analysis): Catch &SYNTAX-ERROR and convert as a warning of the
  appropriate type.

* module/system/base/message.scm (%warning-types)[format]: Handle
  `syntax-error' warnings.

* test-suite/tests/tree-il.test
  ("warnings")["conditionals"]("unterminated", "unexpected ~;",
  "unexpected ~]"): New tests.
  ["unterminated ~{...~}"]: New test.