Better case coverage error message for alias and range enum (#12913)

tests/casestmt/tincompletecaseobject2.nim

@@ -0,0 +1,23 @@
+discard """
+cmd: "nim check $file"
+errormsg: "not all cases are covered; missing: {A, B}"
+nimout: '''
+tincompletecaseobject2.nim(16, 1) Error: not all cases are covered; missing: {B, C, D}
+tincompletecaseobject2.nim(19, 1) Error: not all cases are covered; missing: {A, C}
+tincompletecaseobject2.nim(22, 1) Error: not all cases are covered; missing: {A, B}
+'''
+"""
+type
+  ABCD = enum A, B, C, D
+  AliasABCD = ABCD
+  RangeABC = range[A .. C]
+  AliasRangeABC = RangeABC
+
+case AliasABCD A:
+of A: discard
+
+case RangeABC A:
+of B: discard
+
+case AliasRangeABC A:
+of C: discard

tests/objvariant/tnon_zero_discrim_err.nim

@@ -0,0 +1,14 @@
+discard """
+  errormsg: "low(kind) must be 0 for discriminant"
+  line: 7
+"""
+type
+  HoledObj = object
+    case kind: int
+    of 0: a: int
+    else: discard
+
+let someInt = low(int)
+case someInt
+of 938: echo HoledObj(kind: someInt, a: 1)
+else: discard

tests/objvariant/trt_discrim_err2.nim

@@ -1,10 +1,10 @@
 discard """
-  errormsg: "low(kind) must be 0 for discriminant"
-  line: 7
+  errormsg: " branch initialization with a runtime discriminator only supports ordinal types with 2^16 elements or less."
+  line: 13
 """
 type
   HoledObj = object
-    case kind: int
+    case kind: range[0 .. 20000]
     of 0: a: int
     else: discard