Add logging for exceptions: (#4400)

Log exception messages at several locations.

Previously, these were locations where an exception was caught, but the
exception message was not logged. Logging the exception messages can be
useful for analysis or debugging. The additional logging could have a
small negative performance impact.

Fix #3213.

src/ripple/app/misc/NetworkOPs.cpp

@@ -1162,9 +1162,10 @@ NetworkOPsImp::submitTransaction(std::shared_ptr<STTx const> const& iTrans)
             return;
         }
     }
-    catch (std::exception const&)
+    catch (std::exception const& ex)
     {
-        JLOG(m_journal.warn()) << "Exception checking transaction" << txid;
+        JLOG(m_journal.warn())
+            << "Exception checking transaction " << txid << ": " << ex.what();
 
         return;
     }