feat: add warning for placeholder mips

cloudvolume/datasource/precomputed/metadata.py

@@ -68,6 +68,10 @@ def __init__(
     else:
       self.provenance = self._cast_provenance(provenance)
 
+  def check_for_placeholder_scale(self, mip:int):
+    key = self.key(mip).lower()
+    return 'placeholder' in key
+
   @classmethod
   def create_info(cls, 
     num_channels, layer_type, data_type, encoding, 

cloudvolume/frontends/precomputed.py

@@ -19,7 +19,7 @@
 from ..cacheservice import CacheService
 from .. import exceptions 
 from ..lib import ( 
-  colorize, red, mkdir, 
+  colorize, yellow, red, mkdir, 
   Vec, Bbox, jsonify, BboxLikeType,
 )
 
@@ -55,6 +55,10 @@ def __init__(self,
     self.mip = mip
     self.pid = os.getpid()
 
+    is_placeholder = self.meta.check_for_placeholder_scale(self.mip)
+    if is_placeholder:
+      print(yellow("Warning: The currently selected mip level is marked as a placeholder and likely has no associated image data."))
+
   @property 
   def autocrop(self):
     return self.image.autocrop