msbuild doesn't set non-zero exit code when promoting import warnings to errors with /warnaserror

### Steps to reproduce

a.proj:

```xml

```

b.proj:
```xml

```

Command line
```
msbuild a.proj /warnaserrors
```
### Expected behavior

```
> echo %ERRORLEVEL%
1
```

### Actual behavior

```
Microsoft (R) Build Engine version 15.7.177.53362 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

Build started 5/10/2018 11:05:50 AM.
D:\temp\a.proj(7,3): error MSB4011: "D:\temp\b.proj" cannot be imported again. It was already imported at "D:\temp\a.pr
oj (6,3)". This is most likely a build authoring error. This subsequent import will be ignored.
Project "D:\temp\a.proj" on node 1 (default targets).
T:
Hello
Done Building Project "D:\temp\a.proj" (default targets).

Build FAILED.

D:\temp\a.proj(7,3): error MSB4011: "D:\temp\b.proj" cannot be imported again. It was already imported at "D:\temp\a.
proj (6,3)". This is most likely a build authoring error. This subsequent import will be ignored.

0 Warning(s)
1 Error(s)

> echo %ERRORLEVEL%
0
```

### Environment data
`msbuild /version` output:

```
>msbuild /version
Microsoft (R) Build Engine version 15.7.177.53362 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

15.7.177.53362
```