Create a SpectrumError(Exception)

Create an exception in case of an error due to spectrum problem
eg if spectrum request is not correct
if values are not correct.
use it instead of exit

Signed-off-by: EstherLerouzic <esther.lerouzic@orange.com>

gnpy/core/exceptions.py

@@ -23,3 +23,6 @@ class ServiceError(Exception):
 
 class DisjunctionError(ServiceError):
     '''Disjunction of user-provided request can not be satisfied'''
+
+class SpectrumError(Exception):
+    '''Spectrum errors of the program'''