raise ValueError if usecols has same size as but doesn't exist in headers (pandas-dev#14671)

Author: GGordonGordon

pandas/io/parsers.py

@@ -1425,7 +1425,7 @@ def __init__(self, src, **kwds):
         self.orig_names = self.names[:]
 
         if self.usecols:
-            if len(self.names) > len(self.usecols):
+            if len(self.names) >= len(self.usecols):
                 self.names = [n for i, n in enumerate(self.names)
                               if (i in self.usecols or n in self.usecols)]