Formats of error messages in LSP

[fblanqui]

With the example in #1436 (tests/OK/apply.lp with "require open Stdlib.Prop;" added at the beginning, the displayed error message in VSCode and Emacs is:

file:///home/blanqui/src/lambdapi/tests/OK/apply.lp:1:0-23"/home/blanqui/.opam/5.5.0/.opam-switch/build/lambdapi-stdlib.1.4.0/Prop.lp" cannot be mapped under the library root.
Consider adding a package file under your source tree, or use the [--map-dir] option.

instead of:

file:///home/blanqui/src/lambdapi/tests/OK/apply.lp:1:0-23
"/home/blanqui/.opam/5.5.0/.opam-switch/build/lambdapi-stdlib.1.4.0/Prop.lp" cannot be mapped under the library root.
Consider adding a package file under your source tree, or use the [--map-dir] option.

or perhaps:

tests/OK/apply.lp:1:0-23
"/home/blanqui/.opam/5.5.0/.opam-switch/build/lambdapi-stdlib.1.4.0/Prop.lp" cannot be mapped under the library root.
Consider adding a package file under your source tree, or use the [--map-dir] option.

Now, if I remove "require open Stdlib.Prop;" and replace "constant symbol A : TYPE;" by "constant symbol 0 : TYPE;", I get:

Syntax error. Expected: "non-qualified identifier".

with no position information.