Updating subchart load error to be more descriptive

In the event some malformed folders/files make its way into the
`charts/` directory meant for subcharts, it is possible one may see an
error message like the below:
```
Error: error unpacking foo in app: Chart.yaml file is missing
```

This error is not very descriptive, as it doesn't reference where on
disk `Chart.yaml` is missing. In the event this happens, this error is
being updated to include that `Chart.yaml` is missing for the `foo`
subchart.

Without this updated error messaging, one would need to look into Helm's
codebase to troubleshoot what exactly is wrong.

Signed-off-by: Taylor Jasko <taylor@taylorjasko.com>

cmd/helm/testdata/output/lint-chart-with-bad-subcharts-with-subcharts.txt

@@ -1,8 +1,8 @@
 ==> Linting testdata/testcharts/chart-with-bad-subcharts
 [INFO] Chart.yaml: icon is recommended
-[ERROR] templates/: error unpacking bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
+[ERROR] templates/: error unpacking subchart bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
 [ERROR] : unable to load chart
-	error unpacking bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
+	error unpacking subchart bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
 
 ==> Linting testdata/testcharts/chart-with-bad-subcharts/charts/bad-subchart
 [ERROR] Chart.yaml: name is required

cmd/helm/testdata/output/lint-chart-with-bad-subcharts.txt

@@ -1,7 +1,7 @@
 ==> Linting testdata/testcharts/chart-with-bad-subcharts
 [INFO] Chart.yaml: icon is recommended
-[ERROR] templates/: error unpacking bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
+[ERROR] templates/: error unpacking subchart bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
 [ERROR] : unable to load chart
-	error unpacking bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
+	error unpacking subchart bad-subchart in chart-with-bad-subcharts: validation: chart.metadata.name is required
 
 Error: 1 chart(s) linted, 1 chart(s) failed

pkg/chart/loader/load.go

@@ -174,7 +174,7 @@ func LoadFiles(files []*BufferedFile) (*chart.Chart, error) {
 		case filepath.Ext(n) == ".tgz":
 			file := files[0]
 			if file.Name != n {
-				return c, errors.Errorf("error unpacking tar in %s: expected %s, got %s", c.Name(), n, file.Name)
+				return c, errors.Errorf("error unpacking subchart tar in %s: expected %s, got %s", c.Name(), n, file.Name)
 			}
 			// Untar the chart and add to c.Dependencies
 			sc, err = LoadArchive(bytes.NewBuffer(file.Data))
@@ -194,7 +194,7 @@ func LoadFiles(files []*BufferedFile) (*chart.Chart, error) {
 		}
 
 		if err != nil {
-			return c, errors.Wrapf(err, "error unpacking %s in %s", n, c.Name())
+			return c, errors.Wrapf(err, "error unpacking subchart %s in %s", n, c.Name())
 		}
 		c.AddDependency(sc)
 	}