MD5 verification sanity

We can use switches and enums to make things more understandable
and work with results in a more sensible manner.

PS2: Move file MD5 verification and error handling into
     a separate function to reduce code.

PS3: Default to false in Check_Restore_File_MD5 function

Change-Id: Id3ee0bb444cf13be67bb5dec2c43f7c26a86b954
(cherry picked from commit 5c3950716e5c7c6279efd5f3f338d6d22e5215e2)
diff --git a/twinstall.cpp b/twinstall.cpp
index c2f98fe..50d286c 100644
--- a/twinstall.cpp
+++ b/twinstall.cpp
@@ -296,9 +296,18 @@
 		gui_msg("check_for_md5=Checking for MD5 file...");
 		twrpDigest md5sum;
 		md5sum.setfn(path);
-		int md5_return = md5sum.verify_md5digest();
-		if (md5_return == -2) { // md5 did not match
-			LOGERR("Aborting zip install\n");
+		switch (md5sum.verify_md5digest()) {
+		case MD5_OK:
+			gui_msg(Msg("md5_matched=MD5 matched for '{1}'.")(path));
+			break;
+		case MD5_NOT_FOUND:
+			gui_msg("no_md5=Skipping MD5 check: no MD5 file found");
+			break;
+		case MD5_FILE_UNREADABLE:
+			LOGERR("Skipping MD5 check: MD5 file unreadable\n");
+			break;
+		case MD5_MATCH_FAIL: // md5 did not match
+			LOGERR("Aborting zip install: MD5 verification failed\n");
 			return INSTALL_CORRUPT;
 		}
 	}