Merge: Review errors messages

A big PR but that just review the errors and warning messages printed by the tools.

I tries to follow a standard format:

eg. "Error: cannot find method `foo`."

* start with a message type like "Error:", "Syntax Error:", "Warning:".
  The type is capitalized and terminated by a column.
  The rest on the message starts with a lowercase letter and is terminated with a dot.

* be short and fit on a single line.

* have meaningful information first.
  This helps the reader and remain usable
  when truncated, by an IDE for instance.

* enclose identifiers, keywords and pieces of code with back-quotes.

Do not hesitate no nitpick phrasing or wording or propose better messages.

Close #931

Pull-Request: #1253
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>