<tab> when Sole action warning is not clear enough.

[lewang]

User sees

Sole action: #[(candidate) ^H\203^G^@\302 \207 \207 [marked-candidates candidate helm-marked-candidates] 1]

Maybe a message like this makes it clearer what's going on:

"Sole action. Press to select."

See stackoverflow question:

[thierryvolpiatto]

BTW

#[(candidate) ^H\203^G^@\302 \207 \207 [marked-candidates candidate helm-marked-candidates] 1]

is not an error message but the byte-compiled representation of the anonymous action function of source.
This was happening always in completing-read functions and should be now fixed.
It can happen in other sources that use (action . ,foo) when foo value is #'(lambda (candidate)), but well, it is not a big problem.