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/core_read.cpp

@@ -102,7 +102,7 @@ bool DecodeHexTx(CTransaction& tx, const std::string& strHexTx)
     try {
         ssData >> tx;
     }
-    catch (const std::exception &) {
+    catch (const std::exception&) {
         return false;
     }
 
@@ -119,7 +119,7 @@ bool DecodeHexBlk(CBlock& block, const std::string& strHexBlk)
     try {
         ssBlock >> block;
     }
-    catch (const std::exception &) {
+    catch (const std::exception&) {
         return false;
     }