Doxygen not in path -> more explicit message?

Nobody has claimed this yet.

Doxygen wasn’t in my path... I think we could make the error message more explicit

```
CMake Error at cmake/blt/cmake/SetupDocs.cmake:36 (add_dependencies):
Cannot add target-level dependencies to non-existent target "doxygen_docs".

The add_dependencies works for top-level logical targets created by the
add_executable, add_library, or add_custom_target commands. If you want to
add file-level dependencies see the DEPENDS option of the add_custom_target
and add_custom_command commands.
Call Stack (most recent call first):
src/docs/doxygen/CMakeLists.txt:7 (blt_add_doxygen_target)
```

1. How about making find doxygen _required_?

https://github.com/LLNL/blt/blob/e5564c491cb9032e529768ffc1e7b2696659da92/cmake/thirdparty/SetupThirdParty.cmake#L72

2. Or testing `DOXYGEN_FOUND` when entering the `blt_add_doxygen_target`.

https://github.com/LLNL/blt/blob/e5564c491cb9032e529768ffc1e7b2696659da92/cmake/SetupDocs.cmake#L27

IMO, 1. is legitimate here. But I may be wrong...

Start with cmake/thirdparty/SetupThirdParty.cmake around the Doxygen discovery and cmake/SetupDocs.cmake around blt_add_doxygen_target. Reproduce the failure through src/docs/doxygen/CMakeLists.txt with Doxygen unavailable, then determine whether discovery should be required or the target helper should report DOXYGEN_FOUND. Done means the missing-Doxygen case produces an explicit, actionable CMake error.

Written by the indexing model from the issue text.