Contrary to earfcn, where band can be automatically deduced from earfcn
number because 4G bands never overlap, most functions in nrarfcn accept
as input parameters both nr_arfcn and band, because 5G bands can and do
overlap. As the result it is possible to invoke e.g. dl2ul with
dl_nr_arfcn being outside of downlink spectrum of specified band.

However in b8065120 I've made a thinko and handled such situation with
simple assert which does not lead to useful error feedback from a user
perspective, for example:

    In [2]: xnrarfcn.dl2ul(10000, 1)
    ---------------------------------------------------------------------------
    AssertionError                            Traceback (most recent call last)
    Cell In[2], line 1
    ----> 1 n.dl2ul(10000, 1)

    File ~/src/wendelin/xlte/nrarfcn.py:85, in dl2ul(dl_nr_arfcn, band)
         83 if dl_lo == 'N/A':
         84     raise AssertionError('band%r does not have downlink spectrum' % band)
    ---> 85 assert dl_lo <= dl_nr_arfcn <= dl_hi
         86 ul_lo, ul_hi = nr.get_nrarfcn_range(band, 'ul')
         87 if ul_lo == 'N/A':

    AssertionError:

The issue here is that asserts can be used to only verify internal
invariants, and that reported error does not provide details about which
nrarfcn and band were used in the query.

-> Fix this by providing details in the error reported to incorrect
module usage, and by consistently raising ValueError for "invalid
parameters" cases.

The reported error for above example now becomes

    ValueError: band1: NR-ARFCN=10000 is outside of downlink spectrum