Handle conan.cmake download failures gracefully

This patch adds code to CMakeLists.txt to detect failure to download
conan.cmake and handle it gracefully.

CMake's `file(DOWNLOAD ...)` command [appears to retain][so] an empty or
corrupt file even when the download failed, and it does this silently
without issuing any sort of errors or warnings. Currently, once the
download failed, subsequent configures and reconfigures will all read
from this bad conan.cmake until the build directory is wiped (not even
deleting the cache helps). To fix this, we detect download failures
explicitly, remove the bad file, and report the error to the user.

Also added a SHA256 check to the download command; this is usually
redundant with `TLS_VERIFY ON`, but can be helpful against very bad MITM
actors (sometimes seen in corporate environments).

[so]: https://stackoverflow.com/q/61255773

CMakeLists.txt

@@ -15,7 +15,20 @@ if (NOT EXISTS "${CMAKE_BINARY_DIR}/conan.cmake")
     message(STATUS "Downloading conan.cmake from https://github.com/conan-io/cmake-conan")
     file(DOWNLOAD "https://raw.githubusercontent.com/conan-io/cmake-conan/v0.16.1/conan.cmake"
             "${CMAKE_BINARY_DIR}/conan.cmake"
-            TLS_VERIFY ON)
+            TLS_VERIFY ON
+            EXPECTED_HASH SHA256=396e16d0f5eabdc6a14afddbcfff62a54a7ee75c6da23f32f7a31bc85db23484
+            STATUS conan_download_status_list)
+    list(GET conan_download_status_list 0 conan_download_status)
+    if (conan_download_status)
+        # CMake retains an empty/bad file even if download failed (stackoverflow.com/q/61255773);
+        # if we don't trash it here then we'll be reading a bad conan.cmake when reconfiguring
+        file(REMOVE "${CMAKE_BINARY_DIR}/conan.cmake")
+        list(GET conan_download_status_list 1 conan_download_error_message)
+        message(FATAL_ERROR
+            "Could not download conan.cmake: "
+            "${conan_download_error_message} "
+            "(Error code: ${conan_download_status})")
+    endif ()
 endif ()
 
 include(${CMAKE_BINARY_DIR}/conan.cmake)