Extend the private key stat checking error handling

If the stat operation on the private key failed, the code assumed it
was due to an ENOENT, which may or may not be true. Extend the check
by printing a different error message on non-ENOENT errors for easier
debugging.

Per suggestion by Tom Lane due to an issue with the fairywren animal
in the buildfarm.

Discussion: https://postgr.es/m/1632478.1638305700@sss.pgh.pa.us

src/interfaces/libpq/fe-secure-openssl.c

@@ -1235,9 +1235,14 @@ initialize_SSL(PGconn *conn)
 
 		if (stat(fnbuf, &buf) != 0)
 		{
-			appendPQExpBuffer(&conn->errorMessage,
-							  libpq_gettext("certificate present, but not private key file \"%s\"\n"),
-							  fnbuf);
+			if (errno == ENOENT)
+				appendPQExpBuffer(&conn->errorMessage,
+								  libpq_gettext("certificate present, but not private key file \"%s\"\n"),
+								  fnbuf);
+			else
+				appendPQExpBuffer(&conn->errorMessage,
+								  libpq_gettext("could not stat private key file \"%s\": %m\n"),
+								  fnbuf);
 			return -1;
 		}
 #ifndef WIN32