Propagate JSON parse errors

[StantonMatt]

Summary

• catch exceptions from stdout parser functions before emitting message events
• emit a parseError event and pass the first parse failure to .end() / PythonShell.run() so callers can handle invalid JSON output
• keep user message listener exceptions outside the parser catch path so normal EventEmitter behavior is preserved

Fixes #253.

Validation

• npm test -- --grep "parseError|JSON output"
• npm test
• git diff --check
• review-fix-loop clean after addressing one reviewer finding

[kikiminyes]

I checked the core path locally.

• npm run compileOnce passes.
• I could not run the Python-dependent suite in this Windows environment because Python spawning fails before the tests run, so I validated the changed JS path with a mocked child process instead.
• With invalid JSON on stdout, parseError is emitted, .end() receives PythonShellParseError, and PythonShell.run(..., { mode: "json" }) rejects with the same parse error and empty logs.

The implementation looks directionally correct to me. Keeping parse failures on a separate parseError event avoids Node's special unhandled error event behavior while still surfacing the failure through the completion path.

[PNHD]

The parser/emit ordering on this head is sound, and the focused/full npm suites plus repository Prettier check pass locally on an LF-safe checkout. One required piece is still missing before this is ready as a library API change: the PR exports PythonShellParseError and adds a public parseError event, but README's event documentation does not mention either. For direct PythonShell callers without an .end() callback, parseError is the parser-failure surface; leaving it undocumented can turn the old crash into a failure users do not know how to observe. Please document the new event/class, when it fires, and how .end() / PythonShell.run() receive the first parse failure. I have an overlapping #334 with a different API shape and will defer to maintainer preference rather than requiring either architecture here.