aarch64: Try to avoid inappropriate default errors

aarch64: Try to avoid inappropriate default errors

After parsing a '{' and the first register, parse_typed_reg would
report errors in subsequent registers in the same way as for the
first register.  It used set_default_error, which reports errors
of the form "operand N must be X".

The problem is that if there are multiple opcode entries for the
same mnemonic, there could be several matches that lead to a
default error.  There's no guarantee that the default error for
the register list is the one that will be chosen.

To take an example from the testsuite:

    ext z0.b,{z31.b,z32.b},#0

gave:

    operand 2 must be an SVE vector register

with the error being reported against the single-vector version
of ext, even though the operand is clearly a list.

This patch uses set_fatal_syntax_error to bump the priority of the
error once we're sure that the operand is a list of the right type.