Turn some of the errors back into warnings

compiler/lineinfos.nim

@@ -37,6 +37,8 @@ type
     warnUnusedImportX,
     warnInheritFromException,
     warnEachIdentIsTuple,
+    warnUnsafeSetLen,
+    warnUnsafeDefault,
     warnProveInit, warnProveField, warnProveIndex,
     warnStaticIndexCheck, warnGcUnsafe, warnGcUnsafe2,
     warnUninit, warnGcMem, warnDestructor, warnLockLevel, warnResultShadowed,
@@ -87,6 +89,9 @@ const
     warnUnusedImportX: "imported and not used: '$1'",
     warnInheritFromException: "inherit from a more precise exception type like ValueError, IOError or OSError",
     warnEachIdentIsTuple: "each identifier is a tuple",
+    warnUnsafeSetLen: "setLen can potentially expand the sequence, " &
+                      "but the element type '$1' doesn't have a valid default value",
+    warnUnsafeDefault: "The '$1' type doesn't have a valid default value",
     warnProveInit: "Cannot prove that '$1' is initialized. This will become a compile time error in the future.",
     warnProveField: "cannot prove that field '$1' is accessible",
     warnProveIndex: "cannot prove index '$1' is valid",
@@ -146,6 +151,7 @@ const
     "TypelessParam", "UseBase", "WriteToForeignHeap",
     "UnsafeCode", "UnusedImport", "InheritFromException",
     "EachIdentIsTuple",
+    "UnsafeSetLen", "UnsafeDefault",
     "ProveInit", "ProveField", "ProveIndex",
     "IndexCheck", "GcUnsafe", "GcUnsafe2", "Uninit",
     "GcMem", "Destructor", "LockLevel", "ResultShadowed",