MINOR: errors: further improve parsing error for server-template

MINOR: errors: further improve parsing error for server-template

The following patch improved error messages for server-template lines.
This properly used <tmpl_info.prefix> instead of the <id> field for
them, which prevents to display an empty string.

  94024a60dc197b26a13751a6a0eaa3071f023cfa
  MINOR: server: improve parsing error for server-template

This patch relied on server-template instance detection by checking if
<id> field is NULL. However, this is not appropriate once a
server-template has its ID assigned during _srv_parse_tmpl_init(). The
expanded name is displayed, which can be a misleading information as
this is not present directly in the configuration.

This patch fixes this by now detecting a server-template by checking if
its <tmpl_info.prefix> is non NULL. Thus, a server-template remains
detected as such even after its ID has been generating. Its template
name is used on error messages instead of the expanded prefix.