Add a date+time to the HTML error pages (error_template.html)

[jypeter]

The current HTML error page does not specify a time, and you just get something like

Sorry, Atmosphere_Surface has failed for comparison PMIP4_0k_ref :(
Try the following checks:
[...]

Knowing when the error occurred (and when the page was generated) is a useful information, e.g.

Sorry, Atmosphere_Surface has failed for comparison PMIP4_0k_ref :(

An error occurred on Thu Nov 21 17:07:18 2019

Try the following checks:
[...]

I have just noticed that the comparison pages that are successfully generated don't mention their creation date, so this information should probably also be added

It is probably just a matter of making a small change in share/fp_template/error_template.html

<h1>Sorry, target_component has failed for comparison target_comparison :(</h1>
<p>An error occurred on error_time</p>
<h2>Try the following checks:</h2>

and in share/fp_template/copy_html_error_page.sh

current_date=`date`
sed -i "s/error_time/${error_time}/g" ${atlas_pathfilename}

[jservonnat]

@jypeter ok, please setup a PR