file(DOWNLOAD): Fix message for existing file with matching hash

file(DOWNLOAD STATUS) docs say the return list is of
length 2. However, when an already downloaded file
hash matches EXPECTED_HASH, the return message had
a semicolon that made the return list length 3.
Adjust the message to no longer contain the extra semicolon
and to more clearly describe the situation.

Source/cmFileCommand.cxx

@@ -1914,7 +1914,7 @@ bool HandleDownloadCommand(std::vector<std::string> const& args,
     std::string msg;
     std::string actualHash = hash->HashFile(file);
     if (actualHash == expectedHash) {
-      msg = cmStrCat("returning early; file already exists with expected ",
+      msg = cmStrCat("skipping download as file already exists with expected ",
                      hashMatchMSG, '"');
       if (!statusVar.empty()) {
         status.GetMakefile().AddDefinition(statusVar, cmStrCat(0, ";\"", msg));