Bug#840400: Issue seems in nls_ascii module not available

[Aaron Bugher <aabugher@gmail.com>]

I was hitting this exact error message during installation.  I switched
to another VT and did:

  mkfs.fat /dev/sda1

Then I switched back to the installer VT, and had it try again from
"finalize the partition layout".  Now it's proceeding with no error.

I took the idea from this Ubuntu discussion:

  https://askubuntu.com/questions/502307/

It looks to me like the installer doesn't ensure the EFI partition
contains a filesystem before trying to mount it.

-- 
Aaron Bugher