file(DOWNLOAD EXPECTED_HASH): better error message when download failed

Better error message in case the requested file could not be downloaded. See #24122 (closed) for details.

Test script

cmake_minimum_required(VERSION 3.19)

project(TestTls)
set(VERBOSE ON)

# bad url on purpose
file(DOWNLOAD
    https://example.com/badurl
    output.txt
    EXPECTED_HASH SHA256=b7e13f0b4259e80c3070b583c2f39e53153085a6918718b1c710caf7037572b0
)

set(OUTPUT_FILE ${CMAKE_CURRENT_BINARY_DIR}/output.tar.gz)
file(REMOVE ${OUTPUT_FILE})
file(DOWNLOAD
    https://github.com/census-instrumentation/opencensus-proto/archive/v0.3.0.tar.gz
    ${OUTPUT_FILE}
    # bad hash on purpose
    EXPECTED_HASH SHA256=b7e13f0b4259e80c3070b583c2f39e53153085a6918718b1c710caf7037572b1
    TLS_VERIFY ON
)

Output without this MR

CMake Error at CMakeLists.txt:7 (file):
  file DOWNLOAD HASH mismatch

    for file: [output.txt]
      expected hash: [b7e13f0b4259e80c3070b583c2f39e53153085a6918718b1c710caf7037572b0]
        actual hash: [e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855]
             status: [22;"HTTP response code said error"]



CMake Error at CMakeLists.txt:15 (file):
  file DOWNLOAD HASH mismatch

    for file: [C:/dev-cmake/bug-cmake-download/build/output.tar.gz]
      expected hash: [b7e13f0b4259e80c3070b583c2f39e53153085a6918718b1c710caf7037572b1]
        actual hash: [b7e13f0b4259e80c3070b583c2f39e53153085a6918718b1c710caf7037572b0]
             status: [0;"No error"]

Output with this MR

CMake Error at CMakeLists.txt:7 (file):
  file DOWNLOAD cannot compute hash on failed download

    status: [22;"HTTP response code said error"]


CMake Error at CMakeLists.txt:15 (file):
  file DOWNLOAD HASH mismatch

    for file: [C:/dev-cmake/bug-cmake-download/build/output.tar.gz]
      expected hash: [b7e13f0b4259e80c3070b583c2f39e53153085a6918718b1c710caf7037572b1]
        actual hash: [b7e13f0b4259e80c3070b583c2f39e53153085a6918718b1c710caf7037572b0]

---

Issue: #24122 (closed)