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/math/polynomial/algebraic_numbers.cpp

@@ -1989,7 +1989,7 @@ namespace algebraic_numbers {
                     TRACE("anum_eval_sign", tout << "all variables are assigned to rationals, value of p: " << r << "\n";);
                     return qm().sign(r);
                 }
-                catch (opt_var2basic::failed) {
+                catch (const opt_var2basic::failed &) {
                     // continue
                 }
 

src/math/polynomial/polynomial.cpp

@@ -4493,7 +4493,7 @@ namespace polynomial {
                         }
                         #endif
                     }
-                    catch (sparse_mgcd_failed) {
+                    catch (const sparse_mgcd_failed &) {
                         flet<bool> use_prs(m_use_prs_gcd, false);
                         gcd_prs(u, v, x, r);
                     }