Remove error log when git repo not found

This log was probably more noisy than it needed to be. You can still get
details by turning on debug logs, so this is probably good enough.

This fixes #250.

grgit-gradle/src/main/groovy/org/ajoberstar/grgit/gradle/GrgitPlugin.groovy

@@ -29,7 +29,6 @@ class GrgitPlugin implements Plugin<Project> {
         prj.ext.grgit = grgit
       }
     } catch (Exception e) {
-      project.logger.error("No git repository found for ${project.path}. Accessing grgit will cause an NPE.")
       project.logger.debug("Failed trying to find git repository for ${project.path}", e)
       project.ext.grgit = null
     }