Time::Parser should have a more meaningful error message when input string is too short

[straight-shoota]

Time.parse! "2021-01", "%F" # Error: Unexpected char: '\0' at 7: "2021-01>>"

This error message is misleading. There is not really a \0 character in the input data. The parser just fails at the terminal character of each Crystal string. This is good because it prevents overflow into random data after the end of the string, but the user-facing error message should just state that the input data is too short / didn't fit the expected format.