class_docstr: Raised when input data cannot be merged into a variable font.
code: def _master_name(self, ix):
      if self.merger is not None:
          ttf = self.merger.ttfs[ix]
          if "name" in ttf and ttf["name"].getBestFullName():
              return ttf["name"].getBestFullName()
          elif hasattr(ttf.reader, "file") and hasattr(
              ttf.reader.file, "name"
          ):
              return ttf.reader.file.name
      return f"master number {ix}"
  
imports: import textwrap
label_desc: Use image node_id 3 for calling the VarLibMergeError obj's underlying member method code with example usage: obj._master_name(ix) and returns: str, ix, ttf, ttf