Just for consistency. `proc_macro_error::emit_error` is always called in
the qualified form, but `proc_macro_error::abort!` was sometimes called
in the non-qualified form, which was made possible by
`#[macro_use] external crate proc_macro_error`.

Currently, these tags are not displayed in the rendered view. Instead
of silently doing so, this change introduces a compile error with a
helpful message.

Emit compile-time warnings instead of errors when using doctype and
comment tags within `view!` macro.