`PwOutput.get_output`: improve error message for incorrect `name`

[mbercx]

Running

from qe_tools.outputs import PwOutput

pw_out = PwOutput.from_dir('/Users/mbercx/project/qetools/jupyter/dos_dir/collinear')
structure = pw_out.get_output('incorrect')

only gives a generic KeyError:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[14], [line 4](vscode-notebook-cell:?execution_count=14&line=4)
      1 from qe_tools.outputs import PwOutput
      3 pw_out = PwOutput.from_dir('/Users/mbercx/project/qetools/jupyter/dos_dir/collinear')
----> [4](vscode-notebook-cell:?execution_count=14&line=4) structure = pw_out.get_output('incorrect')

File ~/project/qetools/git-repos/qe-tools/src/qe_tools/outputs/base.py:41, in BaseOutput.get_output(self, name, to)
     28 def get_output(self, name: str, to=None):
     29     """Return a parsed output by key, optionally converted to a library object.
     30 
     31     Args:
   (...)     39         >>> parser.get_output(name="structure", to="pymatgen")
     40     """
---> [41](https://file+.vscode-resource.vscode-cdn.net/Users/mbercx/project/qetools/jupyter/~/project/qetools/git-repos/qe-tools/src/qe_tools/outputs/base.py:41)     output_data = glom(self.raw_outputs, self._output_spec_mapping[name])
     43     if to is None:
     44         return output_data

KeyError: 'incorrect'