verb-pull.md
In dbplyr: A 'dplyr' Back End for Databases

ungroup() produces nice error messages

Code
  memdb_frame(x = 1) %>% pull(non_existent)
Condition
  Error in `pull()`:
  Caused by error:
  ! object 'non_existent' not found
Code
  memdb_frame(x = 1) %>% pull("non_existent")
Condition
  Error in `pull()`:
  ! Can't extract columns that don't exist.
  x Column `non_existent` doesn't exist.
Code
  memdb_frame(x = 1) %>% pull(1000)
Condition
  Error in `pull()`:
  ! Can't extract columns past the end.
  i Location 1000 doesn't exist.
  i There is only 1 column.
Code
  memdb_frame(x = 1) %>% pull(x, "name_non_existent")
Condition
  Error in `pull()`:
  ! Can't extract columns that don't exist.
  x Column `name_non_existent` doesn't exist.