More informative messages for invalid string-value options

[szhorvat]

There should be more informative messages for invalid string-value options.

Example:

IGRealizeDegreeSequence with an invalid method issues "Unknown degree sequence realization method."

The error should list valid methods.

Typically, these errors are issued from C++. The string is converted to an integer with Lookup, taking a default value for which the C++ code reports an error. Instead of Lookup, there should be a helper function that gives a more informative error instead of just returning a default value.