bts: Make f_power_from_band() fail in case of unknown band

Otherwise we get into weird error cases about unbound variables that are hard to debug.

Change-Id: I5716a5dc6ce95daa663be87a1a4fe611b719495e
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index bdd8546..c26afcc 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -2824,6 +2824,9 @@
 		min_dbm_level := {15};
 		max_dbm_level := {30};
 	}
+	case else {
+		Misc_Helpers.f_shutdown(__BFILE__, __LINE__, fail, "Unsupported band: " & band);
+	}
 	}
 
 	return rv;