Better error messages for calling non-callable objects

When available, use the calling code to find a suitable name for what
was being called; this is particularly useful for errors of non-callable
metamethods. This commit also improved the debug information for
order metamethods.

testes/errors.lua

@@ -24,8 +24,9 @@ local function doit (s)
 end
 
 
-local function checkmessage (prog, msg)
+local function checkmessage (prog, msg, debug)
   local m = doit(prog)
+  if debug then print(m) end
   assert(string.find(m, msg, 1, true))
 end
 
@@ -120,6 +121,17 @@ assert(not string.find(doit"a={13}; local bbbb=1; a[bbbb](3)", "'bbbb'"))
 checkmessage("a={13}; local bbbb=1; a[bbbb](3)", "number")
 checkmessage("a=(1)..{}", "a table value")
 
+-- calls
+checkmessage("local a; a(13)", "local 'a'")
+checkmessage([[
+  local a = setmetatable({}, {__add = 34})
+  a = a + 1
+]], "metamethod 'add'")
+checkmessage([[
+  local a = setmetatable({}, {__lt = {}})
+  a = a > a
+]], "metamethod 'lt'")
+
 -- tail calls
 checkmessage("local a={}; return a.bbbb(3)", "field 'bbbb'")
 checkmessage("a={}; do local a=1 end; return a:bbbb(3)", "method 'bbbb'")