install: validate gzip integrity before extracting

This avoids cryptic errors from tarball extraction:

  gzip: stdin: unexpected end of file
  tar: Child returned status 1
  tar: Error is not recoverable: exiting now

By pointing out the cause of the error to the user:

  gzip: Apple_Guides_and_Sample_Code.tgz: unexpected end of file

Author: sunaku

bin/dasht-docsets-install

@@ -93,5 +93,5 @@ for docset; do
   url="https://kapeli.com/feeds/$basename"
 
   wget -P "$DASHT_DOCSETS_DIR" -N -c "$url"
-  dasht-docsets-extract "$tgz"
+  gzip -t "$tgz" && dasht-docsets-extract "$tgz"
 done