Meteor Games staff will usually try to post notice before the downtime so players will have a chance to prepare their crop planting schedule so no crops will wither. They will usually provide notice upon their Facebook discussion or upon their Meteor Games forums.
Other players may also be kind enough to post it upon their Island Paradise related posts (for instance, their Fishing posts or photos) or their Facebook statuses.
There have been instances of emergency downtime where staff have been unable to give notice before the game had to be taken down and fixed or servers upgraded.
A variety of error messages may occur. There may be ones were it simply says, "Oops! This link appears to be broken.", the Oops Chicken, "Just moving some database tables around - we'll have it back up in about 10 minutes, sorry for the inconvenience." or "We are performing maintenance on Island Paradise. We will return shortly."
There is no guarantee how long a down for maintenance may take. It can take the 10 minutes or it can take several hours, for sometimes unexpected errors may be noticed before putting it up and it will take time to fix these. Staff may try to provide updates on how the maintenance is going during the downtime on the the Facebook discussion or Meteor Games forums.
Depending on how long the downtime lasts and what message is placed up, crops may or may not be protected from withering. As a general rule of thumb though, players should not expect their crops to be protected from withering during the downtime. Once the game is back, and if your crops are dead, you can always ask neighbors to revive your crops, or you can purchase a Anti-Withering Totem.
Neopets' error message creature is a sad bird-like creature called a Pteri, and now Island Paradise's error message creature is a startled looking Oops Chicken/Error Chicken/Chicken of Doom.