Write a basic Python script to parse a log file and count the number of specific error messages.
Implement count_error_messages(log_text, error_messages), where log_text is a string containing the file contents and error_messages is a list of distinct strings. Return a dictionary mapping each requested message to its number of non-overlapping occurrences in the complete log text. A message may occur multiple times on one line, and messages with no matches must still appear with count 0.
def count_error_messages(log_text, error_messages):