regulators: Create ActionError class

Create the ActionError exception class.  This exception describes an
error that occurred while executing an action, such as i2c_write_byte.

Change-Id: Ie11b59663ba51da74967454d6ca6ad8856ddd189
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/phosphor-regulators/src/actions/action_error.hpp b/phosphor-regulators/src/actions/action_error.hpp
new file mode 100644
index 0000000..aa502d6
--- /dev/null
+++ b/phosphor-regulators/src/actions/action_error.hpp
@@ -0,0 +1,82 @@
+/**
+ * Copyright © 2020 IBM Corporation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include "action.hpp"
+
+#include <exception>
+#include <string>
+
+namespace phosphor::power::regulators
+{
+
+/**
+ * @class ActionError
+ *
+ * An error that occurred while executing an action.
+ *
+ * This exception describes the action that failed.  If the cause of the failure
+ * was another exception (such as I2CException), the other exception can be
+ * nested inside the ActionError using std::throw_with_nested().
+ */
+class ActionError : public std::exception
+{
+  public:
+    // Specify which compiler-generated methods we want
+    ActionError() = delete;
+    ActionError(const ActionError&) = default;
+    ActionError(ActionError&&) = default;
+    ActionError& operator=(const ActionError&) = default;
+    ActionError& operator=(ActionError&&) = default;
+    virtual ~ActionError() = default;
+
+    /**
+     * Constructor.
+     *
+     * @param action action that was executed
+     * @param error error message
+     */
+    explicit ActionError(const Action& action, const std::string& error = "") :
+        message{"ActionError: " + action.toString()}
+    {
+        if (error.length() > 0)
+        {
+            message += ": ";
+            message += error;
+        }
+
+        // Note: Do not store a reference or pointer to the Action.  It may be
+        // destructed (out of scope) before the exception is caught.
+    }
+
+    /**
+     * Returns the description of this error.
+     *
+     * @return error description
+     */
+    const char* what() const noexcept override
+    {
+        return message.c_str();
+    }
+
+  private:
+    /**
+     * Message describing this exception.
+     */
+    std::string message{};
+};
+
+} // namespace phosphor::power::regulators