Trying to run a library should fail fast

[snprajwal]

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When we try to run a library with dub run, the project is first built, and then errors out saying it is a library.

https://github.com/dlang/dub/blob/13ee9ce38a1c665baefc3bc84cceee27984f5d8a/source/dub/generators/build.d#L697-L699

Would it be better for this to happen right off the bat when the command is executed, and not waste time building the project?