Fix API response parsing in retrieve_historical_rates function

[Javdat]

This commit fixes #60 where convert_currencies() was failing with "subscript out of bounds" error when accessing dat[[8]] in the retrieve_historical_rates function.

Changes:

• Replace hard-coded dat[[8]] references with dat$quotes for accessing exchange rate data
• Update currency pair extraction to handle new API format (e.g., "USDJPY" instead of indexed fields)
• Add proper error handling for API failures with informative error messages
• Use named field access throughout to make code resilient to future API changes

The exchangerate.host API response structure has changed from using numeric indices to named fields. This update ensures the package works with the current API format while being more maintainable for future
changes.

Fixes the error reported when running:

convert_currencies(
  price_start = c(100, 80, 9200, 90),
  from = c("USD", "EUR", "JPY", "USD"),
  to = c("EUR", "USD", "USD", "JPY"),
  date = lubridate::ymd(c(20200601, 20200609, 20200614, 20200623))
)

Tested with R 4.5.0 and priceR 1.0.2.