Include single-line context in syntax errors

[Jesper Öqvist]

JastAdd currently gives no context when reporting a syntax error, it only specifies a file path and line/column location. It would be very useful if the syntax error message included at least the line on which the syntax error occurred.

[Jesper Öqvist]

Improve syntax error messages

Include a single line of context in the error messages for aspect syntax errors.

fixes #230 (bitbucket)

→ <<cset ef4f3e2b9a27>>