More MD5 verification error handling

Distinguish between skipped md5 verification cases:
 - .md5/.md5sum file not found
 - .md5/.md5sum file unreadable

Move MD5 error messages to twrpDigest

Change-Id: I912e3d9ebe64e78bed76eab8aa468d619b45972f
diff --git a/twinstall.cpp b/twinstall.cpp
index d95efed..4247cf7 100644
--- a/twinstall.cpp
+++ b/twinstall.cpp
@@ -260,14 +260,10 @@
 	gui_print("Installing '%s'...\nChecking for MD5 file...\n", path);
 	md5sum.setfn(strpath);
 	md5_return = md5sum.verify_md5digest();
-	if (md5_return == -2) {
-		// MD5 did not match.
-		LOGERR("Zip MD5 does not match.\nUnable to install zip.\n");
+	if (md5_return == -2) { // md5 did not match
+		LOGERR("Aborting zip install\n");
 		return INSTALL_CORRUPT;
-	} else if (md5_return == -1) {
-		gui_print("Skipping MD5 check: no MD5 file found.\n");
-	} else if (md5_return == 0)
-		gui_print("Zip MD5 matched.\n"); // MD5 found and matched.
+	}
 
 #ifndef TW_OEM_BUILD
 	DataManager::GetValue(TW_SIGNED_ZIP_VERIFY_VAR, zip_verify);