Improve SEGY_INVALID_SORTING message

'unable to find sorting' exception is also raised if user doesn't
compare file iline/xline bytes position with segyio.open default values
for iline, xline arguments. Updated possible solutions might help users
catch that faster, and error message might help users catch that.

README.md

@@ -604,6 +604,7 @@ just a collection of traces in arbitrary order, this would fail.
 
 #### Possible solutions
 
+Check if segyio.open `iline` and `xline` input parameters are correct for current file.
 Segyio supports files that are just a collection of traces too, but has to be
 told that it's ok to do so. Pass `strict = False` or `ignore_geometry = True`
 to `segyio.open` to allow or force unstructured mode respectively. Please note

python/segyio/segyio.cpp

@@ -93,6 +93,11 @@ PyObject* RuntimeError( int err ) {
     return RuntimeError( msg.c_str() );
 }
 
+template< typename T1, typename T2 >
+PyObject* RuntimeError( const char* msg, T1 t1, T2 t2 ) {
+    return PyErr_Format( PyExc_RuntimeError, msg, t1, t2 );
+}    
+
 PyObject* IOErrno() {
     return PyErr_SetFromErrno( PyExc_IOError );
 }
@@ -841,7 +846,10 @@ struct metrics_errmsg {
                                    "or offset (%i) field", il, xl, of );
 
             case SEGY_INVALID_SORTING:
-                return RuntimeError( "unable to find sorting." );
+                return RuntimeError( "unable to find sorting." 
+                                    "Check iline, (%i) and xline (%i) "
+                                    "in case you are sure the file is "
+                                    "a 3D sorted volume", il, xl);
 
             default:
                 return Error( err );