Fix undesired exception (ValueError) on getitem of instance CharsetMatches

[Ousret]

One could raise an unwanted exception upon a CharsetMatches instance.

from charset_normalizer.api import from_path

if __name__ == "__main__":

    r = from_path("./data/sample.1.ar.srt")

    r["MacCyrilliX"]

In the current conditions, this would raise ValueError instead of KeyError.