Raise Error on misspelled fragment.

[venthur]

E.g. only 'egg' and 'subdirectory' are allowed.

Closes: #11878

I probably got the usage of DiagnosticPipError, can someone please check?

[sbidoul]

Since the presence of unknown fragments do no harm but are likely a user error, would it be better (safer?) to show a warning instead of raising an error?