`dotnet-symbol` always exits with success code

The exit code of `dotnet-symbol` should be non-zero when it encounters an error. Example:

```
$ cat /foo/bar
cat: /foo/bar: No such file or directory
$ echo $?
1
$ dotnet symbol --symbols /foo/bar
Downloading from http://msdl.microsoft.com/download/symbols/
ERROR: Could not find a part of the path '/foo'.
$ echo $?
0
```