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/db.lua

@@ -823,8 +823,10 @@ assert(a + 30000 == "add" and a - 3.0 == "sub" and a * 3.0 == "mul" and
        -a == "unm" and #a == "len" and a & 3 == "band")
 assert(a|3 == "bor" and 3~a == "bxor" and a<<3 == "shl" and a>>1 == "shr")
 assert (a==b and a.op == "eq")
-assert (a>=b and a.op == "order")
-assert (a>b and a.op == "order")
+assert (a>=b and a.op == "le")
+assert ("x">=a and a.op == "le")
+assert (a>b and a.op == "lt")
+assert (a>10 and a.op == "lt")
 assert(~a == "bnot")
 
 do   -- testing for-iterator name