Add configuration error to Exception hierarchy

docs/developer.md

@@ -129,6 +129,8 @@ Below is an outline of exception inheritance hierarchy:
 + FreqtradeException
 |
 +---+ OperationalException
+|   |
+|   +---+ ConfigurationError
 |
 +---+ DependencyException
 |   |

freqtrade/exceptions.py

@@ -12,6 +12,12 @@ class OperationalException(FreqtradeException):
     """
 
 
+class ConfigurationError(OperationalException):
+    """
+    Configuration error. Usually caused by invalid configuration.
+    """
+
+
 class DependencyException(FreqtradeException):
     """
     Indicates that an assumed dependency is not met.