Stop processing modules when one fails.

This change causes Blueprint to stop calling GenerateBuildActions on modules
when one such call results in errors being generated.  If a module has failed
for some reason and another module depends on it, then processing the dependee
could end up trying to use the failed module, which may be in an invalid state.

Change-Id: I5cc165b7f4e169f90e9570ec7b2a5f9baac2895a

blueprint/context.go

@@ -726,6 +726,9 @@ func (c *Context) generateModuleBuildActions(config interface{},
 		for _, dep := range info.directDeps {
 			if !visited[dep] {
 				walk(dep)
+				if len(errs) > 0 {
+					return
+				}
 			}
 		}
 
@@ -755,6 +758,9 @@ func (c *Context) generateModuleBuildActions(config interface{},
 	for _, module := range c.modules {
 		if !visited[module] {
 			walk(module)
+			if len(errs) > 0 {
+				break
+			}
 		}
 	}