I happened to stumble upon this problem for 3 different releases of Ubuntu on two different computers. While most of the time the swap-partition was mounted despite the warning message, sometimes the GUI refused to start upon boot completion because of it.
Here's what I did as a workaround:
Open a terminal by pressing Ctrl + Alt + T.
gksudo gedit /etc/fstab and press Enter.
Search for the line that looks something like this:
/dev/mapper/cryptswap swap swap defaults 0 0
Now enter the value
noauto right after
defaults (separated by a comma), so it looks like this:
/dev/mapper/cryptswap swap swap defaults,noauto 0 0
Save and exit. This way, your swap partition won't be mounted while booting, thus the warning message won't appear or even hinder the login screen to appear.
Now type in a terminal
gksudo gedit /etc/rc.local
and enter the following lines before the entry
If there's no entry
exit 0, you'll have to enter it right beneath those two lines.
Save and exit again. This will tell your system to wait 10 seconds after login to mount your encrypted swap partition.
Reboot your system. You should not get the warning message any longer. After login, wait some time, then open up a terminal again and type
Your output should look something like this:
fuzzyq@Samsung-R710:~$ free -m
total used free shared buffers cached
Mem: 3949 3806 143 0 86 1783
-/+ buffers/cache: 1936 2013
Swap: 4095 0 4095
The last line is the important one. If there's a positive value showing for total swap, your swap partition was being mounted successfully.
Please note: In your special case, you'd have to repeat the steps for your
/dev/mapper/encriptado /encriptado ext4 defaults 0 0 partition to make this workaround do its magic.