beesh: Abort if download fails

Currently, bee doesn't detect the situation when a partial file is
downloaded. An incomplete archive usually makes the following extract to
fail, but the broken file is kept in the cache an needs to be removed
manually. Only the the situation, when a failed download produced an
empty file is explicitly handled.

Currently, bee contains explicit code to ignore errors from wget.  This
code existed in the initial commit 067ccf2 ("initial revision") from
2010. The probably reason for it is, that the option `--no-globber` is
used to avoid downloading a file, which already exists, a second time.
However, wget terminates with a non-zero exit status in that case.

Handle the case of an existing file in the calling code, so that we can
treat any wget error as a real error and abort (by the `-e` setting of
the shell).

While we are at it, change code to download into a tempfile and rename
it in the success case to avoid problems with partial downloads. This is
more robust than the EXIT trap used by current code.

src/beesh.sh.in

@@ -274,27 +274,22 @@ function fetch_one_file() {
             nocheck=""
         fi
 
-        if [ ! -s "${F}/${file}" ] ; then
-            rm -vf "${F}/${file}"
-        fi
-
-        trap "rm -f ${F}/${file}" EXIT
-
-        print_info "fetching $url"
-        wget \
-            --output-document="${F}/${file}" \
-            --no-clobber \
-            --timeout=60 \
-            --tries=1 \
-            "${url}" || true
-
-        trap - EXIT
-
-        ls -ld "${F}/${file}"
-
-        if [ ! -s "${F}/${file}" ] ; then
-            print_error "ERROR: ${F}/${file} is empty, download failed."
-            exit 1
+        if [ -s "${F}/${file}" ]; then
+            print_info "File ‘${F}/${file}‘ already there; not retrieving."
+        else
+            print_info "fetching $url"
+            wget \
+                --output-document="${F}/${file}.tmp" \
+                --timeout=60 \
+                --tries=1 \
+                "${url}"
+            ls -ld "${F}/${file}.tmp"
+            if [ ! -s "${F}/${file}.tmp" ] ; then
+                print_error "ERROR: ${F}/${file}.tmp is empty, download failed."
+                rm "${F}/${file}.tmp"
+                exit 1
+            fi
+            mv "${F}/${file}.tmp" "${F}/${file}"
         fi
     fi