Short and narrow answer
You need to set the variable case-fold-search to nil in order
to have case sensitive search. So: put this somewhere in your init file:
(setq case-fold-search nil)
See the manual node
on
Searching and Case for
more details.
Some background explanation
spacemacs relies heavily on evil, which emulates Vim. In
general, you should not expect it to accept all (or even most) of
Vim's syntax. :set noignorecase is straight out of Vim, but
there is no set command in evil's limited set of ex
commands.
The reason you got the error messages you did is (I think) because
evil tries to match partially completed commands. In browsing
through the list of ex commands evil emulates (hit C-h v
evil-ex-commands to see the list), the only command that starts
with set is:
("set-initial-state" . evil-ex-set-initial-state)
So, in effect, you tried to tell Emacs to set evil's initial
state to noignorecase -- which doesn't exist, and is not at all what you intended to do.