better error message if app/module that imports from DR (for start/stop) is loaded into process under DR

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on September 10, 2013 12:14:58_

While we don't plan to officially support running a start/stop-using app or lib under DR, this scenario may come up, especially in a systemwide injection model. We should at least detect and provide a better error message than the nested-DR assert here:

LD_LIBRARY_PATH="/usr/local/google/home/pag/Code/dynamorio-git/build/lib64/debug:/usr/local/google/home/pag/Code/dynamorio-git/build/ext/lib64/debug:" /usr/local/google/home/pag/Code/dynamorio-git/build/bin64/drrun -debug -early -root /usr/local/google/home/pag/Code/dynamorio-git/build/ -code_api -- /usr/local/google/home/pag/Code/dynamorio-git/build_suite/build_debug-internal-64/suite/tests/bin/api.startstop

```
\
\
\<(1+x) Handling our fault in a TRY at 0x00000000712a3a60>
\
\
\
\
\
```

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1266_