Clarify Series errors in README

[iHiD]

Not set on the wording, or which cases we should cover, but putting this here so it can be riffed on by maintainers.

Forum discussion: http://forum.exercism.org/t/series-problem-description-unclear/12477

[glennj]

Perhaps

The series length `n` must not be longer than the string of digits.
Trying to extract a 6-digit series from a 5-digit string should result in an error.

[tasxatzial]

Old PR. Should we move it forward?

Simply by looking at the canonical data, it appears that there are even more cases where an error is expected:

• slice length cannot be zero
• slice length cannot be negative
• empty series is invalid

So, it's best to either mention all cases explicitly or remove any reference to errors from the description altogether, which seems more reasonable to me.