feat: clearer fatal error messages for missing API keys and images

[rasros]

What

Improves the top-level failure messages in main.py so users get an actionable line instead of a terse or leaky one, and standardizes the prefix.

API keys — the auto path previously printed just CRITICAL: No API key found. with no hint about what to set. It now names all three env vars:

Error: no LLM API key found. Set one of OPENAI_API_KEY, ANTHROPIC_API_KEY, or GEMINI_API_KEY in your environment.

The explicit-provider path names the specific missing var and the fix:

Error: --provider anthropic was selected but ANTHROPIC_API_KEY is not set. Export ANTHROPIC_API_KEY or pick a provider whose key is set.

Missing image — the generic handler used to print the raw OS error (FATAL: [Errno 2] No such file or directory: 'x.png'). A dedicated FileNotFoundError handler now prints a clean line:

Error: input image not found: x.png

Prefix — standardized CRITICAL: / FATAL: to a single Error: prefix, matching the Error: ... voice already used by the argparse validation in cli.py.

Notes

The image ordering fix (validating before the output directory tree is created) is the follow-up task and lands in a separate PR; this one is message-quality only.

Tests

New test_main.py covers the no-key and missing-key error paths (message names the vars, exit code 1, Error: prefix) plus provider-priority and explicit-model resolution. Full suite: 361 passed.