gdb/ui-out.h: Fix exception handling in do_with_buffered_output

gdb/ui-out.h: Fix exception handling in do_with_buffered_output

Replace throw with throw_exeception in do_with_buffered_output.

This patch fixes regressions in gdb.dwarf2/dw2-dir-file-name.exp
caused by commit 519d63439.

do_with_buffered_output needs to use throw_exception instead of
throw to ensure that exceptions of the correct type are thrown.
If using throw, gdb_exception_error may be wrongly converted into
gdb_exception during print_frame_info.  This prevents the exception
from being caught in print_stack_frame.