Better goto-instrument error reporting for missing files

In the past, goto-instrument was failing with the default
--help error message if it didn't get the correct number
of positional arguments, making it unnecessarily hard to
understand what was failing and why (since the help message
is long and it also didn't have any particular signaling for
what went wrong). This makes it so that it reports it as an
invalid user input exception. Fixes diffblue#2861.

Author: NlightNFotis

src/goto-instrument/goto_instrument_parse_options.cpp

@@ -16,6 +16,7 @@ Author: Daniel Kroening, [email protected]
 #include <memory>
 
 #include <util/config.h>
+#include <util/exception_utils.h>
 #include <util/exit_codes.h>
 #include <util/json.h>
 #include <util/string2int.h>
@@ -824,6 +825,14 @@ int goto_instrument_parse_optionst::doit()
       else
         return CPROVER_EXIT_SUCCESS;
     }
+    else if(cmdline.args.size() < 2)
+    {
+      throw invalid_command_line_argument_exceptiont(
+        "Invalid number of positional arguments passed",
+        "[in] [out]",
+        "goto-instrument needs one input and one output file, aside from other "
+        "flags");
+    }
 
     help();
     return CPROVER_EXIT_USAGE_ERROR;