8297000: [jib] Add more friendly warning for proxy issues

Reviewed-by: erikj, ihse

bin/jib.sh

@@ -128,6 +128,15 @@ install_jib() {
             exit 1
         fi
     fi
+    # Want to check the filetype using file, to see if we got served a HTML error page.
+    # This is sensitive to the filename containing a specific string, but good enough.
+    file ${installed_jib_script}.gz | grep "gzip compressed data" > /dev/null
+    if [ $? -ne 0 ]; then 
+        echo "Warning: ${installed_jib_script}.gz is not a gzip file."
+        echo "If you are behind a proxy you may need to configure exceptions using no_proxy."
+        echo "The download URL was: ${jib_url}"
+        exit 1
+    fi
     echo "Extracting JIB bootstrap script"
     rm -f "${installed_jib_script}"
     gunzip "${installed_jib_script}.gz"