refs: specify error for regular refs with `old_target`

refs: specify error for regular refs with `old_target`

When a reference update tries to update a symref, but the ref in
question is actually a regular ref, we raise an error. However the error
raised in this situation is:

  verifying symref target: '<ref>': reference is missing but expected <old-target>

which is very generic and doesn't indicate the mismatch of types. Let's
make this error more specific:

  cannot lock ref '<ref>': expected symref with target '<old-target>': but is a regular ref

so that users have a clearer understanding.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>