[scripts] previous_release: improve failed download error message

before:
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$ contrib/devtools/previous_release.sh -r -b v0.9.5
[...]
gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
------------------------------------------------------------

now:
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$ contrib/devtools/previous_release.sh -r -b v0.9.5
[...]
curl: (22) The requested URL returned error: 404 Not Found
Download failed.
------------------------------------------------------------

Author: theStack

contrib/devtools/previous_release.sh

@@ -139,7 +139,10 @@ pushd "$TARGET" || exit 1
         fi
         URL="https://bitcoin.org/$BIN_PATH/bitcoin-${tag:1}-$PLATFORM.tar.gz"
         echo "Fetching: $URL"
-        curl -O $URL
+        if ! curl -O -f $URL; then
+            echo "Download failed."
+            exit 1
+        fi
         tar -zxf "bitcoin-${tag:1}-$PLATFORM.tar.gz" -C "$tag" --strip-components=1 "bitcoin-${tag:1}"
         rm "bitcoin-${tag:1}-$PLATFORM.tar.gz"
       fi