Improve error message and documents

- fix grammar error
- Add more information for AES_USE_HARDWARE_ONLY
- Improve error message

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/aes.c b/library/aes.c
index d6ecdcc..00ba40c 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -38,7 +38,7 @@
     defined(__aarch64__) && !defined(MBEDTLS_HAVE_ARM64)
 #define MBEDTLS_HAVE_ARM64
 #if !defined(MBEDTLS_AESCE_C) && defined(MBEDTLS_AES_USE_HARDWARE_ONLY)
-#error "MBEDTLS_AES_C defined, but not all prerequisites"
+#error "MBEDTLS_AES_USE_HARDWARE_ONLY defined, but not all prerequisites"
 #endif
 #endif
 
@@ -47,7 +47,7 @@
     !defined(MBEDTLS_HAVE_X86_64)
 #define MBEDTLS_HAVE_X86_64
 #if !defined(MBEDTLS_AESNI_C) && defined(MBEDTLS_AES_USE_HARDWARE_ONLY)
-#error "MBEDTLS_AES_C defined, but not all prerequisites"
+#error "MBEDTLS_AES_USE_HARDWARE_ONLY defined, but not all prerequisites"
 #endif
 #endif
 
@@ -56,7 +56,7 @@
 #define MBEDTLS_HAVE_X86
 
 #if !defined(MBEDTLS_PADLOCK_C) && defined(MBEDTLS_AES_USE_HARDWARE_ONLY)
-#error "MBEDTLS_AES_C defined, but not all prerequisites"
+#error "MBEDTLS_AES_USE_HARDWARE_ONLY defined, but not all prerequisites"
 #endif
 #endif