better parser errors

error_test/parser/control_flow_errors.sui

@@ -0,0 +1,49 @@
+# Control flow errors
+
+# Missing condition in if
+fn test() -> int
+    if
+        42
+    else
+        0
+
+# Missing body in if
+fn test() -> int
+    if true
+    else
+        0
+
+# Missing variable in for loop
+fn test() -> int
+    for in [1, 2, 3]
+        item
+    0
+
+# Missing 'in' keyword in for loop
+fn test() -> int
+    for item [1, 2, 3]
+        item
+    0
+
+# Missing iterable in for loop
+fn test() -> int
+    for item in
+        item
+    0
+
+# Invalid while syntax
+fn test() -> int
+    while
+        42
+
+# Missing parameters in lambda
+fn test() -> int
+    let f = lambda
+        42
+    f()
+
+# Missing closing parenthesis in lambda
+fn test() -> int
+    let f = lambda(x
+        x
+    f(5)