Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/qt/paymentrequestplus.cpp

@@ -22,7 +22,7 @@
 class SSLVerifyError : public std::runtime_error
 {
 public:
-    SSLVerifyError(std::string err) : std::runtime_error(err) { }
+    explicit SSLVerifyError(std::string err) : std::runtime_error(err) { }
 };
 
 bool PaymentRequestPlus::parse(const QByteArray& data)