aarch64: Tweak register list errors

The error for invalid register lists had the form:

  invalid number of registers in the list; N registers are expected at operand M -- `insn'

This seems a bit verbose.  Also, the "bracketing" is really:

  (invalid number of registers in the list; N registers are expected) at operand M

but the semicolon works against that.

This patch goes for slightly shorter messages, setting a template
that later patches can use for more complex cases.