refactor: Enable -Wswitch in exhaustive switch

Also, apply the comment according to the dev notes.

Also, modify the dev notes to give a lambda-wrapped example.

Can be reviewed via --ignore-all-space

Co-authored-by: Lőrinc <pap.lorinc@gmail.com>

src/script/script.cpp

@@ -151,9 +151,8 @@ std::string GetOpName(opcodetype opcode)
 
     case OP_INVALIDOPCODE          : return "OP_INVALIDOPCODE";
 
-    default:
-        return "OP_UNKNOWN";
-    }
+    } // no default case, so the compiler can warn about missing cases
+    return "OP_UNKNOWN";
 }
 
 unsigned int CScript::GetSigOpCount(bool fAccurate) const

src/script/script_error.cpp

@@ -121,7 +121,7 @@ std::string ScriptErrorString(const ScriptError serror)
             return "Script number overflowed or is non-minimally encoded";
         case SCRIPT_ERR_UNKNOWN_ERROR:
         case SCRIPT_ERR_ERROR_COUNT:
-        default: break;
-    }
+            break;
+    } // no default case, so the compiler can warn about missing cases
     return "unknown error";
 }