make if and elseif throw a better error message when the tag body is omitted

[greelgorke]

I discovered a bug by accident, while composing a template:

{% if name %}
  {{name}}
{% elseif %}
  Someone
{% endif %}

i forgot the body of elseif and used up about a half of a day to find out whats wrong. mostly because the parser nor the compiler complains about it, but the compiled function contains a syntax error. at the end the js engine complains with SyntaxError: Unexpected token ) without any sign of location at runtime.

The patch turns this mess to the error at parsetime: No conditional statement provided on line 5.

[paularmstrong]

Should this be an empty string?

{ code: '', result: false }

[greelgorke]

while debugging the value str was undefined. not exactly sure why, but that was, what i've seen.