Try next host after a "cannot connect now" failure.

 Try next host after a "cannot connect now" failure.
 
 If a server returns ERRCODE_CANNOT_CONNECT_NOW, try the next host,
 if multiple host names have been provided.  This allows dealing
 gracefully with standby servers that might not be in hot standby mode
 yet.
 
 In the wake of the preceding commit, it might be plausible to retry
 many more error cases than we do now, but I (tgl) am hesitant to
 move too aggressively on that --- it's not clear it'd be desirable
 for cases such as bad-password, for example.  But this case seems
 safe enough.
 
 Hubert Zhang, reviewed by Takayuki Tsunakawa
 
 Discussion: https://postgr.es/m/BN6PR05MB3492948E4FD76C156E747E8BC9160@BN6PR05MB3492.namprd05.prod.outlook.com