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.

tests/concepts/tmapconcept.nim

@@ -78,7 +78,7 @@ proc getFirstValue[K,V](m : Map[K,V]): V =
   for i in m.valuesSeq:
     return i
 
-  raise newException(RangeError, "no values")
+  raise newException(RangeDefect, "no values")
 
 proc useConceptProcInGeneric[K, V](t: Table[K, V]): V =
   return t.getFirstValue