deal with longer user-defined errors

[florianschanda]

Proposal:

checks T {
   x > 0, "must be positive"
   '''there are excellent reasons for this
      and this string explains in detail why''',
   x
}

LRM change:

check_declaration ::= expression ',' 
                      [ severity ] STRING_message [ STRING_extrainfo ]
                      [ ',' IDENTIFIER_component_name ]