LogInit: provide mappings for LogLevel::{ERROR,WARNING}

doc/mpd.conf.5.rst

@@ -81,6 +81,8 @@ log_level <default, secure, or verbose>
    Suppress all messages below the given threshold.  The following
    log levels are available:
 
+   - :samp:`error`: errors
+   - :samp:`warning`: warnings
    - :samp:`default`: interesting informational messages
    - :samp:`info`: unimportant informational messages
    - :samp:`verbose`: debug messages (for developers and for

src/LogInit.cxx

@@ -103,6 +103,10 @@ parse_log_level(const char *value)
 		return LogLevel::INFO;
 	else if (StringIsEqual(value, "verbose"))
 		return LogLevel::DEBUG;
+	else if (StringIsEqual(value, "warning"))
+		return LogLevel::WARNING;
+	else if (StringIsEqual(value, "error"))
+		return LogLevel::ERROR;
 	else
 		throw FormatRuntimeError("unknown log level \"%s\"", value);
 }