Print a diagnostic if a BUILD file references a source file that does…

… not exist

Testing Done:
Edited a BUILD file, added a reference to a source file that does not exist.

```
diff --git a/src/python/pants/backend/core/BUILD b/src/python/pants/backend/core/BUILD
index 2da9682..8b1e827 100644
--- a/src/python/pants/backend/core/BUILD
+++ b/src/python/pants/backend/core/BUILD
@@ -4,7 +4,7 @@

 python_library(
   name = 'plugin',
-  sources = ['register.py'],
+  sources = ['register.py', 'does-not-exist.py'],
   dependencies = [
     'src/python/pants/backend/core/targets:all',
     'src/python/pants/backend/core/tasks:all',

```

Before this change, when running

```
PANTS_DEV=1 goal test tests/python/pants_test:all
```

the build would end with an error but not give any further information:

```
Exception message: [Errno 2] No such file or directory
```

After this change, you now get a diagnostic message that gives you a hint why the test failed:

```
 ERROR] Failed to copy src/python/pants/backend/core/does-not-exist.py for library PythonLibrary(BuildFileAddress(/Users/zundel/Src/pants/src/python/pants/backend/core/BUILD, plugin))

               FAILURE

Exception message: [Errno 2] No such file or directory
```

Reviewed at https://rbcommons.com/s/twitter/r/1198/

src/python/pants/backend/python/python_chroot.py

@@ -6,6 +6,7 @@
                         print_function, unicode_literals)
 
 from collections import defaultdict
+import logging
 import os
 import shutil
 import sys
@@ -35,6 +36,8 @@
 from pants.util.dirutil import safe_mkdir, safe_rmtree
 
 
+logger = logging.getLogger(__name__)
+
 class PythonChroot(object):
   _VALID_DEPENDENCIES = {
     PrepCommand: 'prep',
@@ -104,12 +107,25 @@ def copy_to_chroot(base, path, add_function):
 
     self.debug('  Dumping library: %s' % library)
     for relpath in library.sources_relative_to_source_root():
-      copy_to_chroot(library.target_base, relpath, self._builder.add_source)
+      try:
+        copy_to_chroot(library.target_base, relpath, self._builder.add_source)
+      except OSError as e:
+        logger.error("Failed to copy {path} for library {library}"
+                     .format(path=os.path.join(library.target_base, relpath),
+                             library=library))
+        raise
 
     for resources_tgt in library.resources:
       for resource_file_from_source_root in resources_tgt.sources_relative_to_source_root():
-        copy_to_chroot(resources_tgt.target_base, resource_file_from_source_root,
-                       self._builder.add_resource)
+        try:
+          copy_to_chroot(resources_tgt.target_base, resource_file_from_source_root,
+                         self._builder.add_resource)
+        except OSError as e:
+          logger.error("Failed to copy {path} for resource {resource}"
+                       .format(path=os.path.join(resources_tgt.target_base,
+                                                 resource_file_from_source_root),
+                               resource=resources_tgt.address.spec))
+          raise
 
   def _dump_requirement(self, req, dynamic, repo):
     self.debug('  Dumping requirement: %s%s%s' % (str(req),