warnaserror not failing the build with exit 1 if only errors present were warnings

When there are no errors in a build, but only warnings that have been promoted to error with **WarnAsError** then the build will not fail with an exit of 1.

```
$ dotnet build --no-incremental /WarnAsError

...

Build FAILED.

....cs(28,1): error SA1507: Code must not contain multiple blank lines in a row [....csproj]
0 Warning(s)
1 Error(s)

Time Elapsed 00:00:01.55
Exit code => 0
```

Given that there are errors in the build, would expect an exit code of 1

Note that if there are genuine errors in the build, the build will fail correctly with the correct exit code

OS: Linux Ubuntu 18.04
dotnet version: 2.2.3-preview