Nicer error when connecting to standby with hot_standby=off

I recently noticed while setting up a test environment that attempting to
connect to a standby running without hot_standby=on results in a fairly
generic error (I believe "the database system is starting up"). I don't
have my test setup running right now, so can't confirm with a repro case at
the moment, but with a little bit of spelunking I noticed that error text
only shows up in src/backend/postmaster/postmaster.c when
port->canAcceptConnections has the value CAC_STARTUP.

Ideally the error message would include something along the lines of "The
server is running as a standby but cannot accept connections with
hot_standby=off".

I wanted to get some initial feedback on the idea before writing a patch:
does that seem like a reasonable change? Is it actually plausible to
distinguish between this state and "still recovering" (i.e., when starting
up a hot standby but initial recovery hasn't completed so it legitimately
can't accept connections yet)? If so, should we include the possibility if
hot_standby isn't on, just in case?

The enum value CAC_STARTUP is defined in src/include/libpq/libpq-be.h,
which makes me wonder if changing this value would result in a wire
protocol change/something the client wants to know about? If so, I assume
it's not reasonable to change the value, but would it still be reasonable
to change the error text?

Thanks,
James Coleman