Add a new pySim-shell program

pySim-prog was nice when there were only 5 parameters on a SIM that we
could program, and where the use case was pretty limited.  Today, we
have SIM/USIM/ISIM cards with hundreds of files and even more parameters
to program.  We cannot add a command line argument for each file to
pySim-prog.

Instead, this introduces an interactive command-line shell / REPL,
in which one can navigate the file system of the card, read and update
files both in raw format and in decoded/parsed format.

The idea is primarily inspired by Henryk Ploatz' venerable
cyberflex-shell, but implemented on a more modern basis using
the cmd2 python module.

See https://lists.osmocom.org/pipermail/simtrace/2021-January/000860.html
and https://lists.osmocom.org/pipermail/simtrace/2021-February/000864.html
for some related background.

Most code by Harald Welte. Some bug fixes by Philipp Maier
have been squashed.

Change-Id: Iad117596e922223bdc1e5b956f84844b7c577e02
Related: OS#4963
diff --git a/pySim/exceptions.py b/pySim/exceptions.py
index 5d30f76..156ec62 100644
--- a/pySim/exceptions.py
+++ b/pySim/exceptions.py
@@ -41,8 +41,13 @@
 class SwMatchError(Exception):
 	"""Raised when an operation specifies an expected SW but the actual SW from
 	   the card doesn't match."""
-	def __init__(self, sw_actual, sw_expected):
+	def __init__(self, sw_actual, sw_expected, rs=None):
 		self.sw_actual = sw_actual
 		self.sw_expected = sw_expected
+		self.rs = rs
 	def __str__(self):
+		if self.rs:
+			r = self.rs.interpret_sw(sw_actual)
+			if r:
+				return "SW match failed! Expected %s and got %s: %s - %s" % (self.sw_expected, self.sw_actual, r[0], r[1])
 		return "SW match failed! Expected %s and got %s." % (self.sw_expected, self.sw_actual)