Error -> Defect for defects (#13908)

* 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.

lib/pure/options.nim

@@ -74,8 +74,8 @@ type
       val: T
       has: bool
 
-  UnpackError* = object of Defect
-
+  UnpackDefect* = object of Defect
+  UnpackError* {.deprecated: "See corresponding Defect".} = UnpackDefect
 
 proc option*[T](val: T): Option[T] =
   ## Can be used to convert a pointer type (`ptr` or `ref` or `proc`) to an option type.
@@ -486,7 +486,7 @@ when isMainModule:
 
       let tmp = option(intref)
       check(sizeof(tmp) == sizeof(ptr int))
-      
+
       var prc = proc (x: int): int = x + 1
       check(option(prc).isSome)
       prc = nil