Unhelpful error message if there are no columns to compare

[robne1982]

Not particularly important, but when I was working on #71 I tried to run the following

df2 <- tibble(col1 = c("cat", "dog", "mouse", "fly"))
df1 <- tibble(col1 = c("cat", "dog", "rat"))

rCompare(df1, df2, keys = "col1")

Clearly this isn't smart, as without col1, there's nothing left to compare.

However, the output isn't clear

Running rCompare...
Coercing input data to data.frame
 Error in if (nrow(DFA) == 0) { : argument is of length zero 

It would be better to catch this and output a friendly error message.