Add prose for reduction rules involving thrown exceptions,

[ioannad]

i.e., prose for some execution steps for THROWadm, CATCHadm, DELEGATEadm.

The parts of this commit that involve DELEGATEadm may change once PR #220 is settled.

Nonetheless, in this commit the typing rule and the reduction rule for DELEGATEadm are both tentatively changed to match the current formal overview.

[aheejin]

Gentle ping :)