MSBuild incorrectly detects error from ignored error in Makefile .

Hello,

We've noticed issue with MSBuild's error detection for Makefile projects that ignore some of the errors. MSBuild **parses console output** for "errors". Simply "echo error :" will trigger failure. We have Makefile project building FFmpeg and it fails because of that. **Exit code of our build command is 0**, but it still fails due to this output:

```
strip.exe : error : the input file 'libavcodec/x86/proresdsp.o' has no sections [{..}.vcxproj]
make: [{...}/ffmpeg/ffbuild/common.mak:83: libavcodec/x86/proresdsp.o] Error 1 (ignored)
```

Notice the `(ignored)` part? That's because this error is expected and should be ignored and it indeed is ignored. (see minus sign)

common.mak:83
```
%.o: %.asm
$(COMPILE_X86ASM)
-$(if $(ASMSTRIPFLAGS), $(STRIP) $(ASMSTRIPFLAGS) $@)
```

Yet MSBuild after parsing first line with error sets build task to fail state.

I think MSBuild's error parser should be more clever in such cases.

```
Microsoft (R) Build Engine 15.1.1012.6693
```
Regards,
Kacper