LibIMAP: Propagate errors from Parser::parse_number()

More error propagation is still needed, we really want the parser to
just crash early when it encounters unexpected input, instead of trying
to carry on like nothing happened.
I think ErrorOr<unsigned> is *much* better than returning (unsigned)-1
to indicate an error ;)

Userland/Libraries/LibIMAP/Parser.h

@@ -32,7 +32,7 @@ private:
 
     bool at_end() { return m_position >= m_buffer.size(); }
 
-    unsigned parse_number();
+    ErrorOr<unsigned> parse_number();
     Optional<unsigned> try_parse_number();
 
     void parse_response_done();