Validate evaluate_files() arguments

[jeremyfowers]

Problem: the benchmark_files() API expects a List[str] as input to its input_files argument. However, if you pass it a string, it treats that string like a list of characters and throws this exception:

Error: ['h', 'o', 'm', 'e', 'a', 'z', 'u', 'r', 'e', 'u', 's', 'e', 'r', 't', 'u', 'r', 'n', 'k', 'e', 'y', 'm', 'l', 'm', 'o', 'd', 'e', 'l', 's', 't', 'r', 'a', 'n', 's', 'f', 'o', 'r', 'm', 'e', 'r', 's', 'g', 'p', 't', 'j', 'p', 'y'] do not exist, please verify if the file(s) exists.

▄██████████████▄▐█▄▄▄▄█▌
██████▌▄▌▄▐▐▌███▌▀▀██▀▀
████▄█▌▄▌▄▐▐▌▀███▄▄█▌
▄▄▄▄▄██████████████

Solution: validate that benchmark_files() arguments are of the expected type.

cc @danielholanda @ramkrishna2910