c++: avoid slicing exception

Newer clang (or was it gcc?) warn about catching exceptions by value
which have virtual methods. This shouldn't have mattered here because
the exception values where not really used, but using a const
reference is better nonetheless.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>

src/syncevo/SyncContext.cpp

@@ -3746,7 +3746,7 @@ SyncMLStatus SyncContext::doSync()
         bool status = true;
         try {
             status = sendSAN (version);
-        } catch (TransportException e) {
+        } catch (const TransportException &e) {
             if (!sanFormat.empty()){
                 throw;
             }
@@ -4590,11 +4590,11 @@ bool SyncContext::checkForScriptAbort(SharedSession session)
         SharedKey contextKey = m_engine.OpenKeyByPath(sessionKey, "/sessionvars");
         bool abort = m_engine.GetInt32Value(contextKey, "delayedabort");
         return abort;
-    } catch (NoSuchKey) {
+    } catch (const NoSuchKey &) {
         // this is necessary because the session might already have
         // been closed, which removes the variable
         return false;
-    } catch (BadSynthesisResult) {
+    } catch (const BadSynthesisResult &) {
         return false;
     }
 }

test/client-test-main.cpp

@@ -445,7 +445,7 @@ int main(int argc, char* argv[])
       }
       ClientTest::shutdown();
       return failed;
-  } catch (invalid_argument e) {
+  } catch (const invalid_argument &e) {
       // Test path not resolved
       std::cout << std::endl
                 << "ERROR: " << e.what()