[docs] adding error specs

[Michael 'Mickey' Lauer]

I want to improve our docs some more by adding error specifications. I have 
something like the following in mind:

1.) A general introduction of the errors as part of an interface, e.g.

<interface name="org.freesmartphone.GSM.SIM">

  <doc:doc>
    <doc:para>
      The SIM object is used to access the Subscriber Identification Module 
(SIM)
      pluggable as a card into the GSM device. Use it to authenticate yourself
      against the GSM network and read/store phonebook entries as well as 
short
      messages from/to the SIM card memory.
    </doc:para>
    <doc:error name="org.freesmartphone.GSM.SIM.NotPresent">
    Raised, when the SIM can not be detected.
    </doc:error>
  </doc:doc>

and then per method we only announce which errors we raise without any further 
explanation:

  <method name="Register">
    <annotation name="org.freedesktop.DBus.GLib.Async" 
value="fso_gsm_network"/>
    <doc:doc>
        <doc:description>Register with any available service 
provider.</doc:description>
        <doc:inote>This maps to the GSM 07.07 command +COPS=0, see 3GPP TS 
07.07 Chapter 7.3.</doc:inote>
      <doc:errors>
         org.freesmartphone.GSM.Device.Timeout
          org.freesmartphone.GSM.SIM.NotPresent
         org.freesmartphone.GSM.Network.NotFound
    </doc:doc>
  </method>


Does that make sense to you? If so, could I talk anyone into making a patch 
against our doctools magic? :) 

Cheers,

:M: