[typescript] improve error message when LS doesn't start correctly

[marcdumais-work]

It can happen that the TypeScript compiler is missing, when running Theia outside the development environment (e.g. docker images from theia-apps). This causes the TS LS to not start correctly.

The message then given to the user is generic and doesn't help much : "Failed to start language server 'TypeScript'"

We should have a message similar to the one for the C/C++ language client, which is more informative:

https://github.com/theia-ide/theia/blob/e0222f81ddb35a2cdd862f8de81078bfe5b8ce4e/packages/cpp/src/browser/cpp-client-contribution.ts#L41-L58

The error message is customized to give the user useful info about the issue. In this case, we could tell the user to make sure they have the compiler installed: npm install -g typescript

[kittaakos]

See: typescript-language-server/typescript-language-server#28

[tsmaeder]

Languages packages are gone.