docs: document API-error exceptions and request timeouts

[bjornars]

Documents the two newest SDK changes in the README, which had no coverage of either behaviour.

What

• sdk: raise NovemException on API errors instead of a print placeholder #236 — API errors now raise. Writes raise NovemException (or a subclass) carrying the server message instead of the old silent print placeholder. New README Error handling section documents the exception hierarchy (all importable from novem.exceptions), a try/except example, and the create-PUT 409 no-op for objects that already exist. Notes that the Plot(...) constructor itself is a create PUT (unless create=False) and can raise.
• fix: add request timeouts to prevent indefinite hangs (#239) #240 — request timeouts. New Timeouts subsection: every call defaults to (10s connect, 2min read), job.run() allows (30s connect, 30min read), and exceeding either raises requests.exceptions.Timeout.

Commits

1. docs: document API-error exceptions and request timeouts — the new content.
2. docs: README style pass for docs-site conventions — aligns the README with the docs-site style guide (NB: → Note:, fix two comma splices and an already exist typo, add Oxford commas, recast paired-em-dash asides, keep single em-dashes where they fit).

Docs-only; no code changes.