Improve error message for cmake failure (#71404)

I ran into this error as a newbie, was educated
[here](https://discord.com/channels/636084430946959380/642426447167881246/1171016269387018310).
Updating the error message to be more descriptive about the problem and
my options.

llvm/cmake/modules/LLVMProcessSources.cmake

@@ -113,7 +113,12 @@ function(llvm_check_source_file_list)
           else()
               set(fn_relative "${fn}")
           endif()
-          message(SEND_ERROR "Found unknown source file ${fn_relative}
+          message(SEND_ERROR "Found erroneous configuration for source file ${fn_relative}
+LLVM's build system enforces that all source files are added to a build target, \
+that exactly one build target exists in each directory, \
+and that this target lists all files in that directory. \
+If you want multiple targets in the same directory, add \
+PARTIAL_SOURCES_INTENDED to the target specification, though it is discouraged.
 Please update ${CMAKE_CURRENT_LIST_FILE}\n")
         endif()
       endif()