Added more tests

tests/errors/TypeMismatch3.q

@@ -0,0 +1,7 @@
+import mem
+
+main() {
+	a := mem.alloc(16)
+	b := a["not a number"]
+	b += 1
+}

tests/errors/TypeMismatch4.q

@@ -0,0 +1,6 @@
+import mem
+
+main() {
+	a := mem.alloc(16)
+	a["not a number"] = 42
+}

tests/errors/UntypedExpression.q

@@ -0,0 +1,6 @@
+main() {
+	a := unknown()
+	a += 1
+}
+
+unknown() {}

tests/errors_test.go

@@ -57,6 +57,7 @@ var errs = []struct {
 	{"UnknownIdentifier3.q", &errors.UnknownIdentifier{Name: "x"}},
 	{"UnknownPackage.q", &errors.UnknownPackage{Name: "sys"}},
 	{"UnknownStructField.q", &errors.UnknownStructField{StructName: "A", FieldName: "x"}},
+	{"UntypedExpression.q", errors.UntypedExpression},
 	{"UnusedImport.q", &errors.UnusedImport{Package: "sys"}},
 	{"UnusedVariable.q", &errors.UnusedVariable{Name: "x"}},
 	{"VariableAlreadyExists.q", &errors.VariableAlreadyExists{Name: "x"}},