Add basic handling of parse errors to AST linter

refs TryGhost#163
- catch an error from the parse step and push it into the returned messages as a fatal error

Invalid file `test.hbs`:
```hbs
{{#primary_author}}
    {{name}}
{{/author}}
```

Resulting message:
```
[
  {
    "moduleId": "test.hbs",
    "message": "primary_author doesn't match author - 1:3",
    "fatal": true,
    "column": 3,
    "line": 1
  }
]
```

lib/ast-linter/linter.js

@@ -72,7 +72,18 @@ class Linter {
         };
 
         const scanner = this.buildScanner(scannerConfig);
-        const ast = Handlebars.parse(options.source);
+        let ast;
+
+        try {
+            ast = Handlebars.parse(options.source);
+        } catch (err) {
+            addToMessages({
+                message: err.message,
+                fatal: true,
+                column: err.column,
+                line: err.lineNumber
+            });
+        }
 
         scanner.accept(ast);