Use an explicit error message when an error is expected and we don't get one

Previously, if the validation check generating the error in the main code
is removed, the 'should error' tests would just return an empty message

builder/amazon/common/run_config_test.go

@@ -48,7 +48,7 @@ func TestRunConfigPrepare_InstanceType(t *testing.T) {
 	c := testConfig()
 	c.InstanceType = ""
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("err: %s", err)
+		t.Fatalf("Should error if an instance_type is not specified")
 	}
 }
 
@@ -56,14 +56,14 @@ func TestRunConfigPrepare_SourceAmi(t *testing.T) {
 	c := testConfig()
 	c.SourceAmi = ""
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("err: %s", err)
+		t.Fatalf("Should error if a source_ami (or source_ami_filter) is not specified")
 	}
 }
 
 func TestRunConfigPrepare_SourceAmiFilterBlank(t *testing.T) {
 	c := testConfigFilter()
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("err: %s", err)
+		t.Fatalf("Should error if source_ami_filter is empty or not specified (and source_ami is not specified)")
 	}
 }
 
@@ -97,7 +97,7 @@ func TestRunConfigPrepare_EnableT2UnlimitedBadInstanceType(t *testing.T) {
 	c.EnableT2Unlimited = true
 	err := c.Prepare(nil)
 	if len(err) != 1 {
-		t.Fatalf("T2 Unlimited should not work with non-T2 instance types")
+		t.Fatalf("Should error if T2 Unlimited is enabled with non-T2 instance_type")
 	}
 }
 
@@ -110,7 +110,7 @@ func TestRunConfigPrepare_EnableT2UnlimitedBadWithSpotInstanceRequest(t *testing
 	c.SpotPriceAutoProduct = "Linux/UNIX"
 	err := c.Prepare(nil)
 	if len(err) != 1 {
-		t.Fatalf("T2 Unlimited cannot be used in conjuntion with Spot Price requests")
+		t.Fatalf("Should error if T2 Unlimited has been used in conjuntion with a Spot Price request")
 	}
 }
 
@@ -118,7 +118,7 @@ func TestRunConfigPrepare_SpotAuto(t *testing.T) {
 	c := testConfig()
 	c.SpotPrice = "auto"
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("spot_price_auto_product should be set when spot_price is set to auto")
+		t.Fatalf("Should error if spot_price_auto_product is not set and spot_price is set to auto")
 	}
 
 	// Good - SpotPrice and SpotPriceAutoProduct are correctly set
@@ -129,7 +129,7 @@ func TestRunConfigPrepare_SpotAuto(t *testing.T) {
 
 	c.SpotPrice = ""
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("spot_price should be set to auto when spot_price_auto_product is set")
+		t.Fatalf("Should error if spot_price is not set to auto and spot_price_auto_product is set")
 	}
 }
 
@@ -166,7 +166,7 @@ func TestRunConfigPrepare_UserData(t *testing.T) {
 	c.UserData = "foo"
 	c.UserDataFile = tf.Name()
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("err: %s", err)
+		t.Fatalf("Should error if user_data string and user_data_file have both been specified")
 	}
 }
 
@@ -178,7 +178,7 @@ func TestRunConfigPrepare_UserDataFile(t *testing.T) {
 
 	c.UserDataFile = "idontexistidontthink"
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("err: %s", err)
+		t.Fatalf("Should error if the file specified by user_data_file does not exist")
 	}
 
 	tf, err := ioutil.TempFile("", "packer")