Add options to join missing error?

[nilshg]

Currently we have an ArgumentError when matchmissing==:error and there are missings. Almost every time I run into this I have to look up what the other options are called. Could we change the error from:

ArgumentError: Missing values in key columns are not allowed when matchmissing == :error. `missing` found in column :x in left data frame. 

to:

ArgumentError: Missing values in key columns are not allowed when matchmissing == :error. `missing` found in column :x in left data frame. To perform a join with missing values choose `matchmissing = :equal` or `:notequal`. 

or similar?

[bkamins]

Sure - I will add it