[Core] Ignore class load error while class scanning (#1844)

In practice the class path may be rather dirty and not all available classes
can be loaded. Because Cucumber will by default scan in the root package it
guaranteed to run into class loading errors. By logging these at debug level
rather then throwing an exception Cucumber will most probably continue to work
as expected. 

To debug occurrences of missing glue you can enable debug logging via: 
```
-Djava.util.logging.config.file=path/to/logging.properties
```

```properties
handlers=java.util.logging.ConsoleHandler
.level=FINE
java.util.logging.ConsoleHandler.level=FINE
java.util.logging.ConsoleHandler.formatter=java.util.logging.SimpleFormatter
``` 

Fixes: #1843

Author: mpkorstanje

core/README.md

@@ -3,21 +3,40 @@ Cucumber Core
 
 Provides components needed to discover, parse and execute feature files. The
 core is designed with a few extension systems and plugin points. You
-typically don't depend on `cucumber-core` but rather use the different
+typically don't depend directly on `cucumber-core` but rather use the different
 sub modules together e.g. `cucumber-junit` and `cucumber-java`.     
 
 ## Backend ##
 
 Backends consists of two components, a `Backend` and `ObjectFactory`. They are
 respectively responsible for discovering glue classes, registering step definitions
-and creating instances of said glue classes.
+and creating instances of said glue classes. Backend and object factory
+implementations are discovered via SPI
 
 ## Plugin ##
 
 By implementing the Plugin interface classes can listen to execution events
-inside Cucumber JVM.
+inside Cucumber JVM. Consider using a Plugin when creating test execution reports.
 
 ## FileSystem ##
 
 Cucumber uses `java.nio.fileFileSystems` to scan for features and will be able
-to scan features on any file system registered with the JVM.
+to scan features on any file system registered with the JVM.
+
+## Logging ##
+Cucumber uses the Java Logging APIs from `java.util.logging`. See the
+[LogManager](https://docs.oracle.com/javase/8/docs/api/java/util/logging/LogManager.html)
+for configuration options or use the [JUL to SLF4J Bridge](https://www.slf4j.org/legacy.html#jul-to-slf4j).
+
+For quick debugging run with:  
+
+```
+-Djava.util.logging.config.file=path/to/logging.properties
+```
+
+```properties
+handlers=java.util.logging.ConsoleHandler
+.level=FINE
+java.util.logging.ConsoleHandler.level=FINE
+java.util.logging.ConsoleHandler.formatter=java.util.logging.SimpleFormatter
+```

core/src/main/java/io/cucumber/core/resource/ClasspathScanner.java

@@ -1,5 +1,8 @@
 package io.cucumber.core.resource;
 
+import io.cucumber.core.logging.Logger;
+import io.cucumber.core.logging.LoggerFactory;
+
 import java.net.URI;
 import java.nio.file.Path;
 import java.util.ArrayList;
@@ -19,6 +22,8 @@
 
 public final class ClasspathScanner {
 
+    private static final Logger log = LoggerFactory.getLogger(ClasspathScanner.class);
+
     private static final String CLASS_FILE_SUFFIX = ".class";
     private static final String PACKAGE_INFO_FILE_NAME = "package-info" + CLASS_FILE_SUFFIX;
     private static final String MODULE_INFO_FILE_NAME = "module-info" + CLASS_FILE_SUFFIX;
@@ -95,7 +100,7 @@ private Function<Path, Consumer<Path>> processClassFiles(String basePackageName,
                     .filter(classFilter)
                     .ifPresent(classConsumer);
             } catch (ClassNotFoundException | NoClassDefFoundError e) {
-                throw new IllegalArgumentException("Unable to load " + fqn, e);
+                log.debug(e, () -> "Failed to load class " + fqn);
             }
         };
     }