shell: fix double syntax errors aborting program

Type e.g. "e 1 + 2 * 3". The first "+" throws a syntax error
(parentheses are needed around expressions) and the "*" another one
during the final exhaust_until_separator phase, which wasn't protected
by a try/catch.

shell/main.cpp

@@ -424,7 +424,15 @@ int main(int argc, char **argv)
         }
 
         /* Exhaust command input (if not all used by the command) */
-        parser.exhaust_until_separator();
+        while(true) {
+            try {
+                parser.exhaust_until_separator();
+                break;
+            }
+            catch(std::exception &e) {
+                FxOS_log(ERR, "%s", e.what());
+            }
+        }
     }
 
     /* Save command history */