ash: only catch unexpected exceptions in PS1 expansion

ash: only catch unexpected exceptions in PS1 expansion

Commit d1a2fa2a4 (ash: catch error in arithmetic expansion in PS1)
catches all exceptions raised by expandarg().  Some exceptions, such as
the EXEXIT raised when command expansion is used, are expected:

   export PS1='$(echo "$ ")'

These should be processed normally or the shell hangs at the prompt.

function                                             old     new   delta
expandstr                                            344     353      +9
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(add/remove: 0/0 grow/shrink: 1/0 up/down: 9/0)                 Total: 9 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>