Fix a warning about catching a polymorphic exception type by value

Fix a warning about catching a polymorphic exception type by value

```
decafsigners.cc: In member function ‘virtual bool DecafED25519DNSCryptoKeyEngine::verify(const string&, const string&) const’:
decafsigners.cc:140:11: warning: catching polymorphic type ‘class decaf::CryptoException’ by value [-Wcatch-value=]
  140 |   } catch(CryptoException) {
      |           ^~~~~~~~~~~~~~~
decafsigners.cc: In member function ‘virtual bool DecafED448DNSCryptoKeyEngine::verify(const string&, const string&) const’:
decafsigners.cc:276:11: warning: catching polymorphic type ‘class decaf::CryptoException’ by value [-Wcatch-value=]
  276 |   } catch(CryptoException) {
      |           ^~~~~~~~~~~~~~~
```