Make try_from() return the actual error with the partition table.

[AVee]

The try_from() function used to just try both binary and csv partition tables, which resulted in a situation where it's unknown which of the two produced a sensible error.

By checking for the magic bytes 0xAA 0x50 at the start we can know which type of file we are dealing with and parse that type, and return errors accordingly. It also makes Error::InvalidPartitionTable obsolete as it was only used in this specific scenario.

Errors from espflash now look like this:

Error:   × Failed to parse partition table
  ╰─▶ Partition with type 'data' and subtype 'ota' must have size of 0x2000 (8k) bytes

Should resolve #20

[jessebraham]

Very sorry for taking so long to get to this. This is a much better approach IMO. Thanks!