Improve nuxeo-platform-spreadsheet-web build: more verbosity and fail faster

The nuxeo-platform-spreadsheet-web module build went in timeout, apparently frozen during Bower install:

00:33:32.509 [INFO] --- maven-antrun-plugin:1.7:run (gulp-build) @ nuxeo-platform-spreadsheet-web ---
00:33:32.509 [INFO] Executing tasks
(...)
00:33:32.512 init:
(...)
00:35:57.280      [exec] bower zeroclipboard#>=2.1.6             install zeroclipboard#2.3.0
03:20:04.700 Build timed out (after 200 minutes). Marking the build as failed.
03:20:04.705 Build was aborted

  <target name="init" description="Download all node and web dependencies"
          depends="clean-cache">
    <exec executable="${cmd.npm}" failonerror="true">
      <arg value="install" />
    </exec>
    <exec executable="${cmd.bower}" failonerror="true">
      <arg value="install" />
    </exec>
  </target> 

 There are possible improvements for readability and faster fail:

• Output commands before execution (echo)
• the Ant exec task should define a timeout
  https://ant.apache.org/manual/Tasks/exec.html
• Bower install should use interactive mode (is it already the case?)
  Using bower install --config.interactive=false or export CI=true
• Add verbosity to bower and npm, either by default, on developer request (for instance tied with Maven -X option) or when an error happens and the command is automatically launched a second time.
  The last suggestion may rather be moved to a dedicated ticket.
  Be careful not to fill the disk with trace logs when there are no errors.