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/txdb.cpp

@@ -129,7 +129,7 @@ bool CCoinsViewDB::GetStats(CCoinsStats &stats) const {
                 ss << VARINT(0);
             }
             pcursor->Next();
-        } catch (std::exception &e) {
+        } catch (const std::exception& e) {
             return error("%s : Deserialize or I/O error - %s", __func__, e.what());
         }
     }
@@ -218,7 +218,7 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
             } else {
                 break; // if shutdown requested or finished loading block index
             }
-        } catch (std::exception &e) {
+        } catch (const std::exception& e) {
             return error("%s : Deserialize or I/O error - %s", __func__, e.what());
         }
     }