Improve invalid port error message for services

Related to #3681

If a user specifies an invalid port *label* when using
address_mode=driver they'll get an error message about the label being
an invalid number which is very confusing.

I also added a bunch of testing around Service.AddressMode validation
since I was concerned by the linked issue that there were cases I was
missing. Unfortunately when address_mode=driver is used there's only so
much validation that can be done as structs/structs.go validation never
peeks into the driver config which would be needed to verify the port
labels/map.

command/agent/consul/client.go

@@ -1145,10 +1145,13 @@ func getAddress(addrMode, portLabel string, networks structs.Networks, driverNet
 		// If port isn't a label, try to parse it as a literal port number
 		port, err := strconv.Atoi(portLabel)
 		if err != nil {
-			return "", 0, fmt.Errorf("invalid port %q: %v", portLabel, err)
+			// Don't include Atoi error message as user likely
+			// never intended it to be a numeric and it creates a
+			// confusing error message
+			return "", 0, fmt.Errorf("invalid port label %q: port labels in driver address_mode must be numeric or in the driver's port map", portLabel)
 		}
 		if port <= 0 {
-			return "", 0, fmt.Errorf("invalid port: %q: port 0 is invalid", portLabel)
+			return "", 0, fmt.Errorf("invalid port: %q: port must be >0", portLabel)
 		}
 
 		return driverNet.IP, port, nil

command/agent/consul/unit_test.go

@@ -1468,6 +1468,7 @@ func TestGetAddress(t *testing.T) {
 		ExpectedPort int
 		ExpectedErr  string
 	}{
+		// Valid Configurations
 		{
 			Name:      "ExampleService",
 			Mode:      structs.AddressModeAuto,
@@ -1529,6 +1530,8 @@ func TestGetAddress(t *testing.T) {
 			ExpectedIP:   "10.1.2.3",
 			ExpectedPort: 7890,
 		},
+
+		// Invalid Configurations
 		{
 			Name:        "DriverWithoutNetwork",
 			Mode:        structs.AddressModeDriver,