Better error messages from unitfile.erb template.

Have the error messages that unitfile.erb can raise include reference
to the resource (type and title) causing the error, so users can find
where they made the error.

templates/unitfile.erb

@@ -2,7 +2,8 @@
    # Copyright © 2021   Thomas Bellman, Linköping, Sweden
    # Licensed under the GNU LGPL v3+; see the README file for more information.
 -%>
-<% comment_lines = [@comment].flatten.join("\n").split("\n")
+<% resource_ref = "Systemd::%s[%s]" % [ @systemd_resource_type, @title, ]
+   comment_lines = [@comment].flatten.join("\n").split("\n")
    if comment_lines.length() > 0
       comment_lines.each do |cline|
 -%>
@@ -18,10 +19,11 @@
 -%>
 <%= optname %>=
 <%          elsif @systemd_resource_type == 'Unit'
-               raise(Puppet::ParseError, "Bad option name, ``#{optname}''")
+               raise(Puppet::ParseError,
+		     "#{resource_ref}: Bad option name, ``#{optname}''")
             else
                raise(Puppet::ParseError,
-                     'Internal error: $systemd_resource_type bad value')
+                     "#{resource_ref}: Internal error: bad value for $systemd_resource_type, ``#{systemd_resource_type}''")
 -%>
 <%          end -%>
 <%       end -%>
@@ -30,4 +32,4 @@
 <%       end -%>
 <%    end %>
 <% end -%>
-# Managed by Puppet:  Systemd::<%= @systemd_resource_type %>[<%= @title %>]
+# Managed by Puppet:  <%= resource_ref %>