rpc: check `fopen` return code in dumptxoutset

This change improves the usability of the `dumptxoutset` RPC in two ways,
in the case that an invalid path is passed:
  1. return from the RPC immediately, rather then when the file is first
     tried to be written (which is _after_ calculating the UTXO set hash)
  2. return a proper return code and error message instead of the cryptic
     "CAutoFile::operator<<: file handle is nullptr: unspecified
      iostream_category error" (-1)

0 files changed, 0 insertions, 0 deletions