do not fail when generating documentation

recent pod2man version does not work with wget documentation and make the whole compilation failed
slapos stack: Add new explicit dependencies of slapos.core.
add my signature for trusted download of erp5
Add missing copyright & license header.
Raise UserError instead of Exception.

Buildout output is less scary this way.
version up : libpng 1.6.2 and specify so version.
version up : FreeType 2.5.0.1.
version up : poppler 0.22.5.
version up : webp 0.3.0 and specify so version.
Merge branch 'master' into erp5-component
Merge branch 'erp5' into erp5-component
fixup! do not fail when generating documentation

older pod2man does not recognise --errors.
anyway we should just skip make in doc directory.
rebuild libdmtx and librsvg if libpng so version has changed.
rebuild tesseract if libpng or webp so version has changed.
Merge branch 'erp5-component' into erp5