ldap: handle errors explicitly everywhere instead of simply `die`ing

ldap: handle errors explicitly everywhere instead of simply `die`ing

Most codepaths already have explicit error handling (by the means of
checking the return value), which is essential dead code due to setting
`onerror`.

As LDAP errors might get presented to users due to upcoming changes, the
error location should not be present in these error messages, thus
switch to explicit handling.

Only two calls were missing such explicit handling of errors, so these
are amended as appropriate. Further, some `die`s were missing newlines
at the end of the message, which - again - would cause the error
location to be included.

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>