erasure-code: implement consistent error stream

erasure-code: implement consistent error stream

The error stream in the erasure code path is broken and the error
message is sometime not reported back to the user. For instance the
ErasureCodePlugin::factory method has no error stream: when an error
happens the user is left with a cryptic error code that needs lookup in
the sources to figure it out.

The error stream is made more systematic by:

  * always pass it as ostream *ss (instead of something passing it as
    a reference and sometime as a stringstream)

  * ostream *ss is added to ErasureCodePlugin::factory

  * define the ErasureCodeInterface::init pure virtual. It is
    already implemented by all plugins, only in slightly different
    ways. The ostream *ss is added so the init function has a way to
    report error in a human readable way to the caller, in addition to
    the error code.

The ErasureCodePluginJerasure::init return value was incorrectly ignored
when called from ErasureCodePluginJerasure::factory and now returns when
it fails.

The ErasureCodeLrc::layers_init method is given ostream *ss for error
messages instead of printing them via derr.

The ErasureCodePluginLrc::factory method no longer prints errors via
derr: this workaround is made unnecessary by the ostream *ss argument.

The ErasureCodeShec::init ostream *ss argument is ignored. The
ErasureCodeShec::parse method entirely relies on derr to report errors
and converting it goes beyond the scope of this cleanup. There is a
slight risk of getting it wrong and it deserves a separate commit and
careful and independent review.

The PGBackend, OSDMonitor.{cc,h} changes are only about prototype
changes.

Signed-off-by: Loic Dachary <ldachary@redhat.com>