Further improve error message clarity

This commit further improves error message clarity.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 958c247..765293e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -323,10 +323,10 @@
 endif()
 
 if (NOT EXISTS "${MBEDTLS_FRAMEWORK_DIR}/CMakeLists.txt")
-    if (NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git/")
-        message(FATAL_ERROR "${CMAKE_CURRENT_SOURCE_DIR}/.git/ not found. Please ensure you have downloaded a release version of Mbed TLS from GitHub.")
+    if (EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git/")
+        message(FATAL_ERROR "${MBEDTLS_FRAMEWORK_DIR}/CMakeLists.txt not found (and does appear to be a git checkout). Run `git submodule update --init` from the source tree to fetch the submodule contents.")
     else ()
-        message(FATAL_ERROR "${MBEDTLS_FRAMEWORK_DIR}/CMakeLists.txt not found. Run `git submodule update --init` from the source tree to fetch the submodule contents.")
+        message(FATAL_ERROR "${MBEDTLS_FRAMEWORK_DIR}/CMakeLists.txt not found (and does not appear to be a git checkout). Please ensure you have downloaded the right archive from the release page on GitHub.")
     endif()
 endif()
 add_subdirectory(framework)