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" (https://github.com/pragmagic/vscode-nim/blob/0272a0544ddf147fde98f8a8f2b624b7dcb19eb3/src/nimMain.ts#L195).