Improve error message for unclosed items

[GuillaumeGomez]

Goes from:

error: failed to parse template source
         --> <source attribute>:4:11
       ""
  --> src/main.rs:7:14
   |
7  |       source = r#"
   |  ______________^
8  | | {% if x %}
9  | | {% if x %}
10 | | {% endif %}"#,
   | |_____________^

to:

error: expected `endif` to terminate `if` node, found nothing
         --> <source attribute>:2:2
       " if x == 1 %}\n{% if x == 2 %}\n{% endif %}\n\nbla\n"
  --> src/main.rs:7:14
   |
7  |       source = r#"
   |  ______________^
8  | | {% if x == 1 %}
9  | | {% if x == 2 %}
10 | | {% endif %}
11 | |
12 | | bla
13 | | "#,
   | |__^

[Kijewski]

Good idea!