BC break: odd line/column notation in errors

[andig]

@ingydotnet retesting I'm seeing this difference in the output:

  - Old (v3): yaml: line 2: mapping values are not allowed in this context                                                                                          
  - New (v4-rc4): go-yaml load error in scanner at L2.C6: mapping values are not allowed in this context

I guess this is intentional? However, the Lx.Cy notation is not entirely obvious. Does this warrant a separate issue?

Originally posted by @andig in #333 (comment)