implements https://github.com/nim-lang/RFCs/issues/557
It inserts defect handing into a bare except branch
```nim
try:
raiseAssert "test"
except:
echo "nope"
```
=>
```nim
try:
raiseAssert "test"
except:
# New behaviov, now well-defined: **never** catches the assert, regardless of panic mode
raiseDefect()
echo "nope"
```
In this way, `except` still catches foreign exceptions, but panics on
`Defect`. Probably when Nim has `except {.foreign.}`, we can extend
`raiseDefect` to foreign exceptions as well. That's supposed to be a
small use case anyway.
`--legacy:noPanicOnExcept` is provided for a transition period.
* Error -> Defect for defects
The distinction between Error and Defect is subjective,
context-dependent and somewhat arbitrary, so when looking at an
exception, it's hard to guess what it is - this happens often when
looking at a `raises` list _without_ opening the corresponding
definition and digging through layers of inheritance.
With the help of a little consistency in naming, it's at least possible
to start disentangling the two error types and the standard lib can set
a good example here.
