PR binutils/22875: i860/ELF: Report unsupported relocation types

Complement commit f3185997 ("PR 22875: Stop strip corrupting unknown
relocs"), <https://sourceware.org/ml/binutils/2018-02/msg00445.html>,
and also set the `bfd_error_bad_value' error and report an unsupported
relocation type if a howto lookup fails with the i860 backend, fixing a
confusing `no error' error message and removing a binutils test failure:

failed with: <.../binutils/strip-new: tmpdir/bintest.o: no error>, expected: <.* bad value>
.../binutils/strip-new: tmpdir/bintest.o: no error
FAIL: binutils-all/strip-13

with the `i860-stardent-elf' target.

	bfd/
	* elf32-i860.c (lookup_howto): Add `abfd' parameter.  Set the
	`bfd_error_bad_value' error and call `_bfd_error_handler' on a
	howto lookup failure.
	(elf32_i860_reloc_type_lookup): Adjust `lookup_howto' call
	accordingly.
	(elf32_i860_info_to_howto_rela): Likewise.
	(elf32_i860_relocate_splitn): Likewise.
	(elf32_i860_relocate_pc16): Likewise.
	(elf32_i860_relocate_pc26): Likewise.
	(elf32_i860_relocate_section): Likewise.