Downgrade nm fatal errors to non-fatal

Downgrade nm fatal errors to non-fatal

Many of the fatal errors in nm ought to be recoverable.  This patch
downgrades most of them.  The ones that are left are most likely due
to memory allocation failures.

	* nm.c (print_symdef_entry): Don't bomb with a fatal error
	on a corrupted archive symbol table.
	(filter_symbols): Silently omit symbols that return NULL
	from bfd_minisymbol_to_symbol rather than giving a fatal
	error.
	(display_rel_file): Don't give a fatal error on
	bfd_read_minisymbols returning an error, or on not being able
	to read dynamic symbols for synth syms.
	(display_archive): Downgrade bfd_openr_next_archived_file
	error.
	(display_file): Don't bomb on a bfd_close failure.