Failed Precondition error is missing from MomentoErrorCode docs.

[schwern]

PHP and DotNet both have a FailedPreconditionError in their MomentoErrorCode, but it isn't in our docs.

The error message is quite vague: 'System is not in a state required for the operation's execution'

The GRPC error is...

The operation was rejected because the system is not in a state required for the operation's execution. For example, the directory to be deleted is non-empty or an rmdir operation is applied to a non-directory.

@malandis says there's a way it's used...

The increment operation assumes the value on the server is the string representation of an integer. If the user calls increment on a key where that is not true, we return a failed precondition

How can we help the user understand how to fix it?