add default value for Context param in uno::Exception constructors

and all it's subtypes, which is almost never used, so this allows us to
simplify lots of call sites.

Change-Id: I0b05793ea2bdd1027679f63252d42ce4af89433b
remove boilerplate in UNO Exception constructor calls

Now that we have default values for Exception constructor params,
remove lots of boilerplate code.

Change-Id: I620bd641eecfed38e6123873b3b94aaf47922e74