fix: improve error handling across both MCP servers

[devin-ai-integration]

Summary

Several error paths silently swallowed failures or raised opaque exceptions with no actionable context. This PR fixes them across both servers:

Outlook auth (outlook-mcp/auth.py)

• acquire_token_silent can return {"error": ..., "error_description": ...} without an access_token key — previously caused a raw KeyError. Now checks for "access_token" in result before accessing it, logs the error to stderr, and falls back to device-code flow.
• _save_cache was called before the error check after acquire_token_by_device_flow — a failed auth would persist corrupt cache state. Moved _save_cache after the success guard.

Gmail auth (gmail-mcp/auth.py)

• creds.refresh(Request()) failure (revoked token, network error) propagated as an opaque google.auth.exceptions.RefreshError. Now caught and re-raised as RuntimeError with a message pointing the user to delete the token file and re-authenticate.

HTTP wrappers (gmail.py, graph.py)

• r.json() on a 2xx response with non-JSON body (e.g. HTML from a proxy) threw a contextless JSONDecodeError. Now caught and re-raised with the status code and a prefix of the response body.

Server tools (outlook server.py, gmail server.py)

• outlook_draft: result['id'] after createReply/createForward → raw KeyError if the Graph response lacked id. Now uses .get("id") with an explicit ValueError.
• outlook_send and gmail_send: forward mode accepted an empty to and sent it to the API, producing an opaque backend error. Now validates early with ValueError("'to' is required when forwarding a message").

Link to Devin session: https://app.devin.ai/sessions/74827009bae543b298df6aa924affd05
Requested by: @fafaisland

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring