[2506] (unrelated) cleanup: define ReadError as a class, not a struct.

for consistency; it's awkward to see it as a struct while LexerError as a
.class.

The choice is basically a matter of taste, but I personally think it's
better to be defined as a class as its base is defined as a class, and
these exception structs are not just a trivial set of values.

src/lib/dns/master_lexer.h

@@ -301,7 +301,8 @@ class MasterLexer {
 public:
     /// \brief Exception thrown when we fail to read from the input
     /// stream or file.
-    struct ReadError : public Unexpected {
+    class ReadError : public Unexpected {
+    public:
         ReadError(const char* file, size_t line, const char* what) :
             Unexpected(file, line, what)
         {}