Require minimum signer fee of 5bps

This to avoid the ability to disable the system by simply zeroing out
the signer fee through governance.

solidity/contracts/system/TBTCSystem.sol

@@ -170,6 +170,8 @@ contract TBTCSystem is Ownable, ITBTCSystem, DepositLog {
         external onlyOwner
     {
         require(_signerFeeDivisor > 9, "Signer fee divisor must be greater than 9, for a signer fee that is <= 10%.");
+        require(_signerFeeDivisor < 2000, "Signer fee divisor must be less than 2000, for a signer fee that is > 5bps.");
+
         newSignerFeeDivisor = _signerFeeDivisor;
         signerFeeDivisorChangeInitiated = block.timestamp;
         emit SignerFeeDivisorUpdateStarted(_signerFeeDivisor, block.timestamp);