I use Discord Frame Modifier to do that, it may not be the most memory efficient choice or anything, but I like the interface and it works.
Discord mods ported for WoW 2.0
Download DiscordFrameModifier.zip and DiscordLibrary.zip, install them like normal addons.
When in game type in /dfm to open the options. Scroll down the menu on the left side of the window to find "Error Frames" header then look for "UIErrorsFrame" underneath.
Now that you have it selected look towards the right side of the DFM Options window, look for Frame Location and check off Use. You can now arrows under the Frame Location header to 'nudge' the frame in the clicked direction, pressing the button in the middle puts the frame to the middle of the screen. If you want it to go quicker and know what your doing, you can just type in x and y co-ordinates for the frame underneath the arrows, remember to press enter so the numbers you entered are actually used.
Optionally, you can also change the height of the frame (allowing it to show more than just three messages), look at the top right pane of the DFM Options window, and look for Height, click off use to the right side of Height, then move the slider till the desired number of messages appear.
Theres all sorts of options in here I haven't even touched upon, but most you won't need just for moving the UIErrorsFrame.
Edit: For the second poster, using DFM its very simple to hide the frame completely, just check off "Force Hide" after selecting the UIErrorsFrame.
Sorry, recycled screenshot
The one you want is outlined in light blue.