Improve nvm_download_artifact() process, fix #1291

- Delete broken/checksum not matched local cache
 - More output message.

nvm.sh

@@ -1733,10 +1733,19 @@ nvm_download_artifact() {
     TARBALL_URL="${MIRROR}/${SLUG}.tar.${COMPRESSION}"
   fi
 
+  if [ -r "${TARBALL}" ]; then
+    nvm_err "Local cache found: $(nvm_sanitize_path "${TARBALL}")"
     if nvm_compare_checksum "${TARBALL}" "${CHECKSUM}" >/dev/null 2>&1; then
       nvm_err "Checksums match! Using existing downloaded archive $(nvm_sanitize_path "${TARBALL}")"
-  else
-    nvm_echo "Downloading ${TARBALL_URL}..."
+      nvm_echo "${TARBALL}"
+      return 0
+    fi
+    nvm_compare_checksum "${TARBALL}" "${CHECKSUM}"
+    nvm_err "Checksum check failed!"
+    nvm_err "Removing the broken local cache..."
+    command rm -rf "${TARBALL}"
+  fi
+  nvm_err "Downloading ${TARBALL_URL}..."
   nvm_download -L -C - --progress-bar "${TARBALL_URL}" -o "${TARBALL}" || (
     command rm -rf "${TARBALL}" "${tmpdir}"
     nvm_err "Binary download from ${TARBALL_URL} failed, trying source."
@@ -1753,7 +1762,6 @@ nvm_download_artifact() {
     command rm -rf "${tmpdir}/files"
     return 6
   )
-  fi
 
   nvm_echo "${TARBALL}"
 }