Fail when sketch files cannot be opened during load

Before commit e7d4eaa ([breaking] Refactor codebase to use a single
Sketch struct (#1353)), any sketch file that could not be opened would
report a fatal error, but the behavior was changed to silently skip such
failing files instead.

This restores the original behavior and fails to load a sketch of some
of the contained files (after filtering on file extension, so this only
includes files that are actually intended to be processed), instead of
silently excluding the files (which likely produces hard to explain
compilation errors later).

Author: matthijskooijman

Diff for: arduino/sketch/sketch.go

@@ -115,7 +115,7 @@ func New(path *paths.Path) (*Sketch, error) {
 		// Skip files that can't be opened
 		f, err := p.Open()
 		if err != nil {
-			continue
+			return nil, err
 		}
 		f.Close()
 

Diff for: test/test_compile.py

@@ -217,16 +217,16 @@ def test_broken_symlink(sketch_name, link_name, target_name, expect_error):
         result = run_command(["compile", "-b", fqbn, sketch_path])
 
         if expect_error:
-            expected_error = "Error during build: Can't open sketch: stat {}: ".format(symlink_file)
+            expected_error = "Error during build: Can't open sketch: open {}: ".format(symlink_file)
             assert expected_error in result.stderr
             assert not result.ok
         else:
             assert result.ok
 
-    test_broken_symlink("CompileIntegrationTestSymlinkBrokenIno", "link.ino", "doesnotexist.ino", expect_error=False)
-    test_broken_symlink("CompileIntegrationTestSymlinkBrokenCpp", "link.cpp", "doesnotexist.cpp", expect_error=False)
-    test_broken_symlink("CompileIntegrationTestSymlinkBrokenH", "link.h", "doesnotexist.h", expect_error=False)
-    test_broken_symlink("CompileIntegrationTestSymlinkBrokenJson", "link.json", "doesnotexist.json", expect_error=False)
+    test_broken_symlink("CompileIntegrationTestSymlinkBrokenIno", "link.ino", "doesnotexist.ino", expect_error=True)
+    test_broken_symlink("CompileIntegrationTestSymlinkBrokenCpp", "link.cpp", "doesnotexist.cpp", expect_error=True)
+    test_broken_symlink("CompileIntegrationTestSymlinkBrokenH", "link.h", "doesnotexist.h", expect_error=True)
+    test_broken_symlink("CompileIntegrationTestSymlinkBrokenJson", "link.json", "doesnotexist.json", expect_error=True)
     test_broken_symlink("CompileIntegrationTestSymlinkBrokenXXX", "link.xxx", "doesnotexist.xxx", expect_error=False)