Set type of error expected in try/expect blocks

[pausz]

There are some except clauses without an explicit type of error. Not sure if bare except behaviour was the desired behaviour -- from context doesn't seem like it. May also be useful to print some sort of message in except block.

• sw_app.py _do_RPC() (eg, json decode error, maybe related to Improve RPC error handling #37?)
• sw_datastore.py get()
• sw_datastore.py DataDir
• sw_server.py imports (eg, import error)
• sw_server.py browser (eg, webbrowser error)

[cliffckerr]

Done