modifyKey: error out on nonexisting new primary user id

OpenKeychain/src/main/res/values/strings.xml

@@ -643,6 +643,7 @@
     <string name="msg_mf_error_fingerprint">Actual key fingerprint does not match the expected one!</string>
     <string name="msg_mf_error_keyid">No key ID. This is an internal error, please file a bug report!</string>
     <string name="msg_mf_error_integrity">Internal error, integrity check failed!</string>
+    <string name="msg_mf_error_noexist_primary">Bad primary user id specified!</string>
     <string name="msg_mf_error_revoked_primary">Revoked user ids cannot be primary!</string>
     <string name="msg_mf_error_pgp">PGP internal exception!</string>
     <string name="msg_mf_error_sig">Signature exception!</string>