curate rename: does not work if some fields do not exist

[joverlee521]

Discovered in ebola workflow

If some requested fields in the --field-map do not exist, then some fields might not be renamed as expected.
This is due to the use of field_map.pop while looping through the list

augur/augur/curate/rename.py

Lines 60 to 64 in 23b9fff

	for idx,names in enumerate(field_map[:]):
	old_name, new_name = names
	if old_name not in existing_fields:
	print_err(f"WARNING: Asked to rename field {old_name!r} (to {new_name!r}) but it doesn't exist in the input data.")
	field_map.pop(idx)

The indexes doesn't match, resulting in the wrong elements getting dropped:

my_list = [1,2,3,4,5]
for idx, num in enumerate(my_list[:]):
    if num in {2, 4}:
         my_list.pop(idx)

print(my_list) 
# Expects [1,3,5]
[1, 3, 4]