fixes #8671; show helpful msg (lookup symbol, eg iterator) on 'attempting to call undeclared routine' error (#8786)

tests/errmsgs/undeclared_routime_compiles.nim

@@ -0,0 +1,11 @@
+# D20180828T234921:here
+template foo*(iter: untyped): untyped =
+  when compiles(iter.unexistingField): 0
+  elif compiles(iter.len): 1
+  else: 2
+
+proc foo[A]()=
+  let a2 = @[10, 11]
+  let a3 = foo(pairs(a2))
+
+foo[int]()