AVFormatParameters are converted into corresponding private options in
av_open_input_file/stream() compat wrappers, so accessing them from
demuxers is redundant.
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>
