refactor(ctb): Simplify require statements by nesting in an if()

Rather than using an OR clause we put them inside an if statement. This allows us to

clarify the error message and is more testable.

packages/contracts-bedrock/src/Safe/LivenessModule.sol

@@ -74,10 +74,10 @@ contract LivenessModule is ISemver {
         uint256 ownersCount = SAFE.getOwners().length;
         for (uint256 i = 0; i < _previousOwners.length; i++) {
             ownersCount--;
-            require(
-                ownersCount < MIN_OWNERS || _canRemove(_ownersToRemove[i]),
-                "LivenessModule: the safe still has sufficient owners, or the owner to remove has signed recently"
-            );
+            if (ownersCount >= MIN_OWNERS) {
+                require(_canRemove(_ownersToRemove[i]), "LivenessModule: the owner to remove has signed recently");
+            }
+
             _removeOwner({
                 _prevOwner: _previousOwners[i],
                 _ownerToRemove: _ownersToRemove[i],
@@ -154,10 +154,14 @@ contract LivenessModule is ISemver {
         address[] memory owners = SAFE.getOwners();
         uint256 numOwners = owners.length;
         // Ensure that the safe is not being left in an unsafe state with too few owners.
-        require(
-            numOwners >= MIN_OWNERS || (numOwners == 1 && owners[0] == FALLBACK_OWNER),
-            "LivenessModule: Safe must have the minimum number of owners or be owned solely by the fallback owner"
-        );
+        if (numOwners == 1) {
+            require(owners[0] == FALLBACK_OWNER, "LivenessModule: must transfer ownership to fallback owner");
+        } else {
+            require(
+                numOwners >= MIN_OWNERS,
+                "LivenessModule: must remove all owners and transfer to fallback owner if numOwners < minOwners"
+            );
+        }
 
         // Check that the threshold is correct. This check is also correct when there is a single
         // owner, because get75PercentThreshold(1) returns 1.