Daily Challenge 272 - Printer Errors

In a factory, a printer prints labels for boxes. For one kind of box, the printer has to use colors which, for the sake of simplicity, are named with letters from a to m.

The colors used by the printer are recorded in a control string. For example a "good" control string would be aaabbbbhaijjjm meaning that the printer used three times color a, four times color b, one time color h then one time color a...

Sometimes there are problems: lack of colors, technical malfunction and a "bad" control string is produced e.g. aaaxbbbbyyhwawiwjjjwwm with letters not from a to m.

Write a function printer_error which given a string will output the error rate of the printer as a string representing a rational whose numerator is the number of errors and the denominator the length of the control string. Don't reduce this fraction to a simpler expression.

The string has a length greater or equal to one and contains only letters from a to z.

Examples:
s = "aaabbbbhaijjjm"
error_printer(s) => "0/14"

s = "aaaxbbbbyyhwawiwjjjwwm"
error_printer(s) => "8/22"

Tests:
s = "aaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbmmmmmmmmmmmmmmmmmmmxyz"
s = "aaabbbbbbccccccddddddeee"
s = "verylargepiggoesoink"

Good luck!

This challenge comes from g964 on CodeWars. Thank you to CodeWars, who has licensed redistribution of this challenge under the 2-Clause BSD License!

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