dialyzer: catch parse errors raised from the try/of body in start/1 - #11679

dialyzer_cl_parse:start/0 wrapped argparse:parse/3 in a `try ... of ... catch ...`. A try/of only runs its catch clauses over the guarded expression, not over the `of` body. postprocess_side_effects/1 and dialyzer_options:build/1 ran in the `of` body, so throws raised there escaped the catch:

- the `-pa` clause calls `cl_error/1`, which throws `{dialyzer_cl_parse_error, _};` and
- `dialyzer_options:check_output_plt/1` throws `{dialyzer_error, _}`.

Both escaped as `{nocatch, ...}` past the plain case in `dialyzer:plain_cl/0`, crashing the CLI with an Erlang stack trace and a non-standard exit status instead of the documented graceful "dialyzer: <msg>" + exit code. A common typo error (e.g. `-pa` on a non-existent directory) hit this path.

Move the `case` expression inside the `try` body so the of-body is covered by the `catch`, and add a `throw:{dialyzer_error, Msg}` clause for the `check_output_plt` path. Split `start/0` into a thin wrapper over `start/1` (which takes the argument list) so the parsing path is testable without spawning a node; `start/1` is exported but marked `-doc false` as it is not part of the public API.

Add `dialyzer_cl_SUITE:bad_pa_dir_returns_error/1`, which drives `start/1` with a non-existent `-pa` directory and asserts a clean `{error, _}` return.