generate_ansi_c_start_function reports user errors via return code

Do not throw an exception upon invalid source input. Instead, print an
error message and return `true`.

Author: tautschnig

regression/cbmc/human-readable-error-on-wrong-main-signature/main.c renamed to regression/ansi-c/human-readable-error-on-wrong-main-signature/main.c

@@ -2,7 +2,7 @@ CORE
 main.c
 
 'main' with signature .* found
-^EXIT=6$
+^EXIT=(1|64)$
 ^SIGNAL=0$
 --
 Invariant check failed

regression/cbmc/human-readable-error-on-wrong-main-signature/test.desc renamed to regression/ansi-c/human-readable-error-on-wrong-main-signature/test.desc

@@ -480,15 +480,19 @@ bool generate_ansi_c_start_function(
     {
       const namespacet ns{symbol_table};
       const std::string main_signature = type2c(symbol.type, ns);
-      throw invalid_source_file_exceptiont{
-        "'main' with signature '" + main_signature +
-        "' found,"
-        " but expecting one of:\n"
-        "   int main(void)\n"
-        "   int main(int argc, char *argv[])\n"
-        "   int main(int argc, char *argv[], char *envp[])\n"
-        "If this is a non-standard main entry point please provide a custom\n"
-        "entry function and point to it via cbmc --function instead"};
+      messaget message(message_handler);
+      message.error().source_location = symbol.location;
+      message.error() << "'main' with signature '" << main_signature
+                      << "' found,"
+                      << " but expecting one of:\n"
+                      << "   int main(void)\n"
+                      << "   int main(int argc, char *argv[])\n"
+                      << "   int main(int argc, char *argv[], char *envp[])\n"
+                      << "If this is a non-standard main entry point please "
+                         "provide a custom\n"
+                      << "entry function and use --function instead"
+                      << messaget::eom;
+      return true;
     }
   }
   else