fix: ldap error swallowed

[ezioruan]

This fix will return the origin LDAP error which is fully formated at go-ldap.

[korylprince]

Unfortunately, this change would cause a breaking change in the API. Specifically, supplying incorrect credentials to Bind should not return an error. I will leave this PR open as a possible change for a v4 of the API, but don't find this alone compelling enough to release a new API version.