Add filename when in handlebars errors

[posva]

When handlebars gives an error, it doesn't contain the filename that is actually producing the error.
Should help solve problems like #401

Instead of

Parse error on line 21

We now have

[readme.md] Parse error on line 21

@egoist @zigomir Feel free to modify how the filename is included in the error message

[zigomir]

@posva nice!