Fail nicely when trying to use petsc4py methods without petsc4py available

[connorjward]

This is particularly important for when someone forgets to pass --no-build-isolation when pip installing firedrake (or some other downstream library). If petsc4py isn't in the isolated build environment then you get an unhelpful missing attribute error for petsctools.init, which doesn't explain why init is missing`.

[JHopeCollins]

This is a very good addition, thanks @connorjward.
I just have one suggestion to improve the error message even further, other than that I think it's good to go.

For the record, if I "forget" to pass --no-build-isolation with the current petsctools main branch then I get this error:

  AttributeError: module 'petsctools' has no attribute 'init'
  error: subprocess-exited-with-error
  
  × Getting requirements to build editable did not run successfully.
  │ exit code: 1
  ╰─> See above for output.
  
  note: This error originates from a subprocess, and is likely not a problem with pip.

Whereas if I "forget" using this branch then I get:

  ImportError: Cannot load 'init' from module 'petsctools' because petsc4py is not available
  error: subprocess-exited-with-error
  
  × Getting requirements to build editable did not run successfully.
  │ exit code: 1
  ╰─> See above for output.
  
  note: This error originates from a subprocess, and is likely not a problem with pip.

[JHopeCollins]

I fixed a syntax error in my suggestion. LGTM now.