Remove magic number and add error handling

[JimMadge]

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⤴️ Summary

• Remove magic number
• Add DataSafeHavenActiveDirectoryError exception
• Add error handling for when number of fields is not what is expected

I'm a little ambivalent about the error handling. Checking that the number of columns returned is what we expect is sensible, we are interacting with an external process.

However, this

data-safe-haven/data_safe_haven/administration/users/active_directory_users.py

Lines 44 to 51 in c8e9b7a

	if (
	user.username
	and user.given_name
	and user.surname
	and user.phone_number
	and user.email_address
	and user.country
	):

Is already checked here

data-safe-haven/data_safe_haven/administration/users/user_handler.py

Lines 40 to 51 in c8e9b7a

	for required_field in [
	"GivenName",
	"Surname",
	"Phone",
	"Email",
	"CountryCode",
	]:
	if (not reader.fieldnames) or (
	required_field not in reader.fieldnames
	):
	msg = f"Missing required CSV field '{required_field}'."
	raise ValueError(msg)

🌂 Related issues

Closes #1513

🔬 Tests

[jemrobinson]

As the flow is:

• check we're providing the right arguments
• run an external script with those arguments
• get the output
• check that the output has the right form

I think it's correct to have two checks.

[jemrobinson]

@JimMadge: happy with whichever option you choose to resolve the external Powershell script issue.

[jemrobinson]

LGTM