Supports suffixes such as "kB" and "MB".

PluginUnconfigured exceptions are logged with level "info" instead of
"error".  This suppresses some rather boring messages in the default
log level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/565

These messages can be confusing, but they are not critical.

This is the documented value, but for unknown reasons, "info" was
really the default.

This was never noticed because there are only very few "info" level
messages.

Make LogError()/FormatError() wrappers for those.  Now we can log
exceptions with a lower level.

Prepare for templated functions.

Print all nested exceptions on a single line to avoid confusion.

Propagate the error to the caller instead of logging it.

Preparing to move logger calls out of lower-level libaries, and
propagating error details to the caller instead.

Regression by commit bbdf2dcf