Report unavailable Sphinx themes

A failed theme import was never handled because the fallback branch
repeated the success condition, leaving the component undefined and the
reason unreported.

Mark the component as not found and report the import error message,
without a traceback, unless searching quietly.

Change-Id: Iaa6c5febf93709c5426cf0dc36eff10f7260db54

cmake/FindSphinx.cmake

diff --git a/cmake/FindSphinx.cmake b/cmake/FindSphinx.cmake
index d1488eb..b80fd2d 100644
--- a/cmake/FindSphinx.cmake
+++ b/cmake/FindSphinx.cmake
@@ -95,7 +95,9 @@
       endif (NOT theme_component STREQUAL component)
 
       execute_process (
-        COMMAND ${Python_EXECUTABLE} -c "import ${theme_component}"
+        # Report only the reason of a failed import instead of a traceback.
+        COMMAND ${Python_EXECUTABLE} -c
+          "import sys\ntry:\n  import ${theme_component}\nexcept ImportError as e:\n  sys.exit(str(e))"
         ERROR_STRIP_TRAILING_WHITESPACE
         ERROR_VARIABLE _Sphinx_BUILD_ERROR
         OUTPUT_QUIET
@@ -104,8 +106,10 @@
 
       if (_Sphinx_BUILD_RESULT EQUAL 0)
         set (Sphinx_${component}_FOUND TRUE)
-      elseif (_Sphinx_BUILD_RESULT EQUAL 0)
-        message (WARNING "Could not determine whether Sphinx component '${theme_component}' is available: ${_Sphinx_BUILD_ERROR}")
+      else (_Sphinx_BUILD_RESULT EQUAL 0)
+        if (NOT Sphinx_FIND_QUIETLY)
+          message (STATUS "Sphinx component '${theme_component}' is not available: ${_Sphinx_BUILD_ERROR}")
+        endif (NOT Sphinx_FIND_QUIETLY)
         set (Sphinx_${component}_FOUND FALSE)
       endif (_Sphinx_BUILD_RESULT EQUAL 0)