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/bitcoind.cpp

@@ -97,7 +97,7 @@ bool AppInit(int argc, char* argv[])
         try
         {
             ReadConfigFile(mapArgs, mapMultiArgs);
-        } catch(std::exception &e) {
+        } catch (const std::exception& e) {
             fprintf(stderr,"Error reading configuration file: %s\n", e.what());
             return false;
         }
@@ -147,7 +147,7 @@ bool AppInit(int argc, char* argv[])
         detectShutdownThread = new boost::thread(boost::bind(&DetectShutdownThread, &threadGroup));
         fRet = AppInit2(threadGroup);
     }
-    catch (std::exception& e) {
+    catch (const std::exception& e) {
         PrintExceptionContinue(&e, "AppInit()");
     } catch (...) {
         PrintExceptionContinue(NULL, "AppInit()");