The error message should point which shared function was attempted to call

[vporton]

The error message should point which shared function was attempted to call. Now it's hard to find it.

send capability required, but not available
  (cannot call a `shared` function from a `composite query` function; only calls to `query` and `composite query` functions are allowed)

[crusso]

I guess this is related to #4583?

[vporton]

I guess this is related to #4583?

No, these two bugs are unrelated.