Fix AES-OFB support for errors, tests and self-test

Adds error handling into mbedtls_aes_crypt_ofb for AES errors, a self-test
for the OFB mode using NIST SP 800-38A test vectors and adds a check to
potential return errors in setting the AES encryption key in the OFB test
suite.
diff --git a/tests/Makefile b/tests/Makefile
index e15d945..23baef3 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -46,7 +46,7 @@
 endif
 
 APPS =	test_suite_aes.ecb$(EXEXT)	test_suite_aes.cbc$(EXEXT)	\
-	test_suite_aes.cfb$(EXEXT)		test_suite_aes.ofb$(EXEXT)	\
+	test_suite_aes.cfb$(EXEXT)	test_suite_aes.ofb$(EXEXT)	\
 	test_suite_aes.rest$(EXEXT)	\
 	test_suite_arc4$(EXEXT)		test_suite_asn1write$(EXEXT)	\
 	test_suite_base64$(EXEXT)	test_suite_blowfish$(EXEXT)	\