It is a harmless error -- it tries to load updated microcode if available (compared to what is included with the BIOS). While you could ignore it, it would be best to get the latest microcode (that may fix CPU bugs) and throw it in /lib/firmware/amd-ucode/. See here
for an overview, and here
for a download link (the amd64.org site has been down for quite a while now so you'll have to grab the last available microcode from here
, linked in the above thread). There is an INSTALL file in the tarball with instructions, but basically you just have to create the /lib/firmware/amd-ucode directory and copy microcode_amd.bin and microcode_amd_fam15h.bin to that directory.
If you do that, the error will disappear and any CPU bugs that may have been present should be fixed (not that you were likely to have noticed them without updated microcode anyway).