Generator source validation?

The following scenario occurred (simplified for brevity):

```
# in a jamfile
alias MY_ALIAS ;
lib LIB : MY_ALIAS ;
```

Then, trying to build the `LIB` target:

```
*** argument error
* rule class@archive-generator.run ( project name ? : property-set : sources + )
* called with: ( object(project-target)@3272 LIB : object(property-set)@7510 : )
* missing argument sources
```

The source for the `LIB` target ends up producing an empty source list (user error). I can clearly tell that has happened from the error message, but I can't tell _where_ it occurred.

**Note**: we use the convention of specifying a single `LIB` target per library and in a project that uses multiple libraries, there are multiple `LIB` targets which is why it's not entirely clear _which_ `LIB` target the error message is referring to.

I have a couple of ideas that could be used to help the end user solve this problem.
1. For the base generator class, the `run` signature could be updated to use `*` instead of `+` and then check to see if `sources` is empty and print a better error message (target name, location, etc).
2. The Jam interpreter and the class implementations could be modified to support a "to string" or "repr" method and those would be called in the above error message. This would also be a benefit in several other places like `--debug-building` and `--debug-generators`.

The problem with the second idea is that it doesn't help the Python port.

Thoughts?