Error codes are uint32_t, not unsigned long.

Fix a few remnants of them being unsigned long. Also rename extremely unhelpful
variable names in SSL_get_error. i is now ret_code to match the header.

Change-Id: Ic31d6626bfe09c9e21c03691dfc716c5573833ea
Reviewed-on: https://boringssl-review.googlesource.com/3881
Reviewed-by: Adam Langley <agl@google.com>