dub and source libraries

[drug]

Currently, if you try to build a package that is a source library, dub 
fails:
```bash
$dub build
Main package must not have target type "sourceLibrary". Cannot build.
```
it fails exactly:
```bash
$echo $?
2
```
But is it the error at all? Shouldn't it be like "Nothing to build" (w/o 
any error) instead of "Cannot build"? For example if you try to build 
all packages in a directory you need to check if the package is a source 
library and that really complicates things.