Bug 8573 - Translation difficult in picture-upload.tt due to nested sentence in if...

Bug 8573 - Translation difficult in picture-upload.tt due to nested sentence in if/foreach/if/elsif - construction

Text could not be correctly translated due to poor parsing of nested sentences in Pootle.

*Sentences de-nested to have whole sentence in each if /elsif branch
*Cleaned up <li></li> handling: moved closing </li> outside the for each loop to prevent orphaned closing </li>s.
*Changed indentation for better readibility
*Changed WARNING to ERROR because it is an *Error* during upload.
*Wording simplified (for translation).
*In one case added hint to refer to online help (size).

This way text should be easier to be translated in Pootle.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>