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/net.cpp

@@ -601,7 +601,7 @@ int CNetMessage::readHeader(const char *pch, unsigned int nBytes)
     try {
         hdrbuf >> hdr;
     }
-    catch (const std::exception &) {
+    catch (const std::exception&) {
         return -1;
     }
 
@@ -1068,7 +1068,7 @@ void ThreadMapPort()
                 MilliSleep(20*60*1000); // Refresh every 20 minutes
             }
         }
-        catch (boost::thread_interrupted)
+        catch (const boost::thread_interrupted&)
         {
             r = UPNP_DeletePortMapping(urls.controlURL, data.first.servicetype, port.c_str(), "TCP", 0);
             LogPrintf("UPNP_DeletePortMapping() returned : %d\n", r);
@@ -1854,7 +1854,7 @@ bool CAddrDB::Write(const CAddrMan& addr)
     try {
         fileout << ssPeers;
     }
-    catch (std::exception &e) {
+    catch (const std::exception& e) {
         return error("%s : Serialize or I/O error - %s", __func__, e.what());
     }
     FileCommit(fileout.Get());
@@ -1890,7 +1890,7 @@ bool CAddrDB::Read(CAddrMan& addr)
         filein.read((char *)&vchData[0], dataSize);
         filein >> hashIn;
     }
-    catch (std::exception &e) {
+    catch (const std::exception& e) {
         return error("%s : Deserialize or I/O error - %s", __func__, e.what());
     }
     filein.fclose();
@@ -1914,7 +1914,7 @@ bool CAddrDB::Read(CAddrMan& addr)
         // de-serialize address data into one CAddrMan object
         ssPeers >> addr;
     }
-    catch (std::exception &e) {
+    catch (const std::exception& e) {
         return error("%s : Deserialize or I/O error - %s", __func__, e.what());
     }