src/{lexer-{proto,impl},parser/floc-proto}.lisp: Conditionify parse errors.

src/{lexer-{proto,impl},parser/floc-proto}.lisp: Conditionify parse errors.

  * Introduce condition classes for parse errors.

  * Introduce `classify-condition' to describe the different reportable
    conditions to the user, and change `count-and-report-errors*' to use
    this rather than having special knowledge.  Now it pretty much just
    counts and prints.

  * Move the complicated error-message printing machinery from the
    `syntax-error' and `lexer-error' functions into the condition
    reporting functions.  (Now they don't actually need to be formatted
    until they're just about to be presented to a user.)