Issue Description

The takeoff command in MAVProxy provides misleading feedback that confuses users, especially beginners.

Current Behavior

When executing takeoff 10 in unsuitable conditions:

• Console prints: "Take Off started" ✅ (misleading - just means command sent)
• Terminal shows: "Got COMMAND_ACK: NAV_TAKEOFF: FAILED" ❌ (actual result, often missed)

This happens when:

1. Vehicle is disarmed
2. Copter is in wrong mode (STABILIZE, LAND, RTL, etc.)
3. Invalid altitude provided (zero, negative, or non-numeric)

Expected Behavior

Command should validate conditions before sending and provide clear, immediate feedback:

• "Takeoff refused: Vehicle is DISARMED"
• "Takeoff refused: Current mode is STABILIZE"
• "Error: Invalid altitude value"

Proposed Solution

Add client-side pre-flight validation in mavproxy_cmdlong.py:

1. Check armed status
2. Check flight mode (copter requires GUIDED or AUTO)
3. Validate altitude input
4. Provide actionable error messages

Environment

• MAVProxy version: master branch
• Vehicle type: Copter (SITL)
• Python version: 3.10

Testing

Reproduced in SITL:

STABILIZE> takeoff 10
Take Off started
Got COMMAND_ACK: NAV_TAKEOFF: FAILED

Willing to Contribute

Yes, I have a working implementation tested in SITL and ready for PR.