Catch exceptions by const-reference

Exceptions caught by value incur needless cost in C++, most of them can
be caught by const-reference, especially as nearly none are actually
used. This could allow compiler generate a slightly more efficient code.

src/tactic/probe.cpp

@@ -524,7 +524,7 @@ public:
             }
             return false;
         }
-        catch (found) {
+        catch (const found &) {
             return true;
         }
     }
@@ -554,7 +554,7 @@ public:
             }
             return false;
         }
-        catch (found) {
+        catch (const found &) {
             return true;
         }
     }