Combining all shares of a group throws and error

[grempe]

Steps to reproduce:

1. Generate a 2/3 set of shares.
2. Copy all three shares
3. Paste all three shares into the Combine > Shares text box

Expected result:
The proper secret is decoded, matching the original secret.

Actual result:
An error is immediately shown in the UI:
Error: Wrong number of mnemonic groups. Expected $groupThreshold groups, but undefined were provided.

This also behaves the same with 3/5 split. Where an error is thrown unless exactly three shares are provided.

It should be possible to reconstruct a split secret from all of the shares, not just threshold shares.

[iancoleman]

Yes I agree, where there's excess shares it would be good to use only the required number instead of showing an error. I'll add this feature.

Also related to #6 Improve error messages for share recovery