Debugger: Improve error message when attempt to attach a process which has already attached by another debugger

## Environment data
`dotnet --info` output:
Server Machine:
OS: linuxmint17
VS Code version:1.10.2
C# Extension version: v1.8.0-beta2(https://github.com/OmniSharp/omnisharp-vscode/releases/tag/v1.8.0-beta2)
vsdbg: https://aka.ms/getvsdbgsh

Client Machine:
OS: centos7

## Steps to reproduce
1. Open attached folder with VSCode on Client and Server Machine.
[hwapp.zip](https://github.com/OmniSharp/omnisharp-vscode/files/830433/hwapp.zip)

2. Update the ".NET Core Remote Attach" configuration on launch.json file on Client Machine.
3. Launch hwapp process on Server Machine with VSCode.
4. Remote attached the server process.

## Expected behavior
Is it possible to improve the error message similar like "Attach to process fail, the process has already attached to a debugger".

## Actual behavior
Pop up an error message: