PluginService: be less verbose if injection fails

If a plugin depends on an unavailable service, the context injection
will fail. When a plugin fails to instantiate for whatever reason, we
don't need to see the whole huge stack trace, normally. Let's reserve
that for when debug mode is enabled. But still log the failure.

Author: ctrueden

src/main/java/org/scijava/plugin/DefaultPluginService.java

@@ -239,7 +239,9 @@ public <PT extends SciJavaPlugin> List<PT> createInstances(
 			return p;
 		}
 		catch (final Throwable t) {
-			log.error("Cannot create plugin: " + info, t);
+			final String errorMessage = "Cannot create plugin: " + info;
+			if (log.isDebug()) log.debug(errorMessage, t);
+			else log.error(errorMessage);
 		}
 		return null;
 	}