Tweak image-based difference message (pointfreeco#188)

When in record mode, we include any difference directly in the failure,
which is handy, but in the case of images "Expected snapshot to match
reference" isn't quite right. I think a plain ole description
"Newly-taken snapshot does not match reference" makes more sense.

Sources/SnapshotTesting/Snapshotting/NSImage.swift

@@ -18,8 +18,8 @@ extension Diffing where Value == NSImage {
       guard !compare(old, new, precision: precision) else { return nil }
       let difference = SnapshotTesting.diff(old, new)
       let message = new.size == old.size
-        ? "Expected snapshot to match reference"
-        : "Expected snapshot@\(new.size) to match reference@\(old.size)"
+        ? "Newly-taken snapshot does not match reference"
+        : "Newly-taken snapshot@\(new.size) does not match match reference@\(old.size)"
       return (
         message,
         [XCTAttachment(image: old), XCTAttachment(image: new), XCTAttachment(image: difference)]

Sources/SnapshotTesting/Snapshotting/UIImage.swift

@@ -18,8 +18,8 @@ extension Diffing where Value == UIImage {
       guard !compare(old, new, precision: precision) else { return nil }
       let difference = SnapshotTesting.diff(old, new)
       let message = new.size == old.size
-        ? "Expected snapshot to match reference"
-        : "Expected snapshot@\(new.size) to match reference@\(old.size)"
+        ? "Newly-taken snapshot does not match reference"
+        : "Newly-taken snapshot@\(new.size) does not match match reference@\(old.size)"
       let oldAttachment = XCTAttachment(image: old)
       oldAttachment.name = "reference"
       let newAttachment = XCTAttachment(image: new)