Better error messages when nim toolchain not in $PATH

[mcon]

If nimsuggest isn't in $PATH, then in stead of an error stating this, one gets an infinite loading bar "Nim: check project" (

vscode-nim/src/nimMain.ts

Line 195 in 0272a05

{location: ProgressLocation.Window, cancellable: false, title: 'Nim: check project...'},

).