Fivethirtyeight has explained away the exit poll issue very easily. Due to the way they are collected, they have an inherent bias toward young voters. Young voters are overwhelmingly pro Sanders. This results in the exit polls being biased towards Sanders. If this is the case, the margin of errors are worthless because they assume a representative sample.

The voter suppression thing is certainly a problem, but it is a big leap to say it is only a problem for Sanders or proves anything nefarious.