Fixing #697 with better exception

When including a template which is not defined, the exception raised is
*undefined method `split` for nil:NilClass*

This occurs for a scenario like the following:
`{% include nil %}`
or
`{% include undefined-var %}`

Making the code raise an argument error to allow better understanding of
the include error

lib/liquid/locales/en.yml

@@ -22,3 +22,5 @@
       tag_never_closed: "'%{block_name}' tag was never closed"
       meta_syntax_error: "Liquid syntax error: #{e.message}"
       table_row: "Syntax Error in 'table_row loop' - Valid syntax: table_row [item] in [collection] cols=3"
+    argument:
+      include: "Argument error in tag 'include' - Illegal template name"