Show the invalid string in the error message.
While at it also prefer "Could not" over "Couldn't", plain forms are
preferred over contractions (simplify readability, especially for non
English-savvy people).
Signed-off-by: Anton Khirnov <anton@khirnov.net>
The variable is used for containing the parsed value of framerate,
using a lexically consistent name eases readability/understanding.
Signed-off-by: Anton Khirnov <anton@khirnov.net>