make all catch() arguments const

- I saw this on http://en.cppreference.com/w/cpp/language/try_catch and
  thought it would be a good idea
- also unify used format to better be able to search for exception
  uses in our codebase

src/qt/paymentrequestplus.cpp

@@ -181,8 +181,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
         }
         // TODO: detect EV certificates and set merchant = business name instead of unfriendly NID_commonName ?
     }
-    catch (SSLVerifyError& err)
-    {
+    catch (const SSLVerifyError& err) {
         fResult = false;
         qWarning() << "PaymentRequestPlus::getMerchant : SSL error: " << err.what();
     }