Fix/imf_data_one better not to stop, but go next iteration

Thank you for merging my previous pull request. This is another one. Download for all countries, like the example below, sometimes fails. As imf_data_one function does for loop to transact 60 countries by each, the last iteration may have small number of countries and those countries may not have requested indicator or frequency. I would like to propose imf_data_one function will not stop, but go next iteration, when it finds unavailable indicator or frequency.

[failed example]

imf_data(database_id = "IFS",
               indicator = "GG_GALM_G01_XDC",
               country = "all",
               freq = "A",
               start = 1900, end = current_year()
)

christophergandrud / imfr

Fix/imf_data_one better not to stop, but go next iteration #20