Make the message about name not referring to a user-defined type more generic

test/libsolidity/syntaxTests/errors/error_as_function_param.sol

@@ -1,4 +1,4 @@
 error E(uint);
 function f(E x) pure returns (uint) {}
 // ----
-// TypeError 5172: (26-27): Name has to refer to a struct, enum or contract.
+// TypeError 5172: (26-27): Name has to refer to a user-defined type.

test/libsolidity/syntaxTests/errors/using_2.sol

@@ -9,4 +9,4 @@ contract C {
     }
 }
 // ----
-// TypeError 5172: (91-92): Name has to refer to a struct, enum or contract.
+// TypeError 5172: (91-92): Name has to refer to a user-defined type.

test/libsolidity/syntaxTests/errors/weird3.sol

@@ -4,4 +4,4 @@ contract C {
     E x;
 }
 // ----
-// TypeError 5172: (29-30): Name has to refer to a struct, enum or contract.
+// TypeError 5172: (29-30): Name has to refer to a user-defined type.

test/libsolidity/syntaxTests/errors/weird4.sol

@@ -6,4 +6,4 @@ contract C {
     }
 }
 // ----
-// TypeError 5172: (64-65): Name has to refer to a struct, enum or contract.
+// TypeError 5172: (64-65): Name has to refer to a user-defined type.