Fixing diagnostics

[Alidra]

This PR addresses issue #1274

• Messages in the diagnostics are made a bit more expressive.
• The diagnostic on the ";" at the end of symbol introduction is removed.
• location of the diagnostic is kept as is on the server side. On the client side (specifically in Emacs) the displayed range is shortened to the three first characters at most of the initial range sent by the server .

[ciaran-matthew-dunne]

Hi, many thanks for the work on this!

I will test and give my comments when I arrive back to Paris on Monday.