Fixes based on PR to tidy up code and improve error messages

Fixes based on PR feedback to improve code clarity, use of
correct lookup function, and improve error output.

Author: TGWDB

regression/cbmc/enum_is_in_range/enum_test10.desc

@@ -3,7 +3,7 @@ enum_test10.c
 
 ^EXIT=6$
 ^SIGNAL=0$
-^file enum_test10.c line \d+ function main: __enum_is_in_range expects enum argument type$
+^file enum_test10.c line \d+ function main: __CPROVER_enum_is_in_range expects enum argument type$
 --
 ^\[main.assertion.1\] line \d+ assertion __CPROVER_enum_is_in_range\(i\): SUCCESS$
 ^\[main.assertion.1\] line \d+ assertion __CPROVER_enum_is_in_range\(i\): FAILURE$

regression/cbmc/enum_is_in_range/enum_test3.c

@@ -15,7 +15,7 @@ int main()
   enum my_enum ev2;
   ev1 = first;
   ev2 = fifth;
-  while(!(ev1 == ev2))
+  while(ev1 != ev2)
   {
     ev1++;
     assert(__CPROVER_enum_is_in_range(ev1));

src/ansi-c/c_typecheck_expr.cpp

@@ -2847,12 +2847,9 @@ exprt c_typecheck_baset::do_special_functions(
     if(expr.arguments().size() != 1)
     {
       error().source_location = expr.source_location();
-      error() << "__enum_is_in_range expects one argument" << eom;
-      std::ostringstream error_message;
-      error_message << expr.source_location().as_string() << ": "
-                    << "__enum_is_in_range expects one argument, "
-                    << expr.arguments().size() << " were provided";
-      throw invalid_source_file_exceptiont{error_message.str()};
+      error() << identifier << " expects one argument, " << 
+        expr.arguments().size() << " were provided" << eom;
+      throw 0;
     }
     // Replace expr with giant boolean that checks all the values
     // of the enumeration as disjunction
@@ -2863,8 +2860,7 @@ exprt c_typecheck_baset::do_special_functions(
       auto c_enum_tag_type =
         type_try_dynamic_cast<c_enum_tag_typet>(arg1.type()))
     {
-      symbolt enum_type = this->lookup(c_enum_tag_type->get_identifier());
-      const c_enum_typet &c_enum_type = to_c_enum_type(enum_type.type);
+      const c_enum_typet &c_enum_type = follow_tag(*c_enum_tag_type);
       const c_enum_typet::memberst enum_values = c_enum_type.members();
 
       std::vector<exprt> disjuncts;
@@ -2879,10 +2875,9 @@ exprt c_typecheck_baset::do_special_functions(
     else
     {
       // Can't enum range check a non-enum
-      std::ostringstream error_message;
-      error_message << expr.source_location().as_string() << ": "
-                    << "__enum_is_in_range expects enum argument type";
-      throw invalid_source_file_exceptiont{error_message.str()};
+      error().source_location = expr.source_location();
+      error() << identifier << " expects enum argument type" << eom;
+      throw 0;
     }
   }
   else if(