tools:iio:iio_utils: add error handling

Add error handling to calls which can indicate a major problem by
returning an error code.
This also sets ret to -ENOENT in iioutils_get_type() and
iioutils_get_param_float() to indicate if no matching directory entry was
found.
Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>