webadmin: tidy up unavailable page slightly

Currently the page displayed when certain features are unavailable
has badly formatted HTML: the title contains markup and the message
is displayed as a heading.

Target: master
Patch: https://rb.dcache.org/r/7425/
Acked-by: Albert Rossi
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6