Add descriptive revert reasons to DataEscrowV3 contract

[crtahlin]

Description

Contract reverts with empty data instead of helpful error messages, making debugging very difficult.

Current Behavior

Error: execution reverted (no data present; likely require(false) occurred)
Error data: 0x

Expected Behavior

Descriptive revert reasons like:

• "Seller not registered as agent"
• "Invalid expiry: must be 1-365 days"
• "Amount below minimum threshold"
• "Content hash cannot be zero"
• "Key commitment cannot be zero"

Impact

• Developers waste time guessing what validation failed
• Support burden increases
• Integration testing is painful

Suggested Implementation

Replace:

require(someCondition);

With:

require(someCondition, "Descriptive error message");

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Found during seller flow UX testing - contract rejected call with no explanation