AP_KDECAN: regularise CAN pre-arm failure messages

AP_Arming tacks on the sub-system bit.

Remove PiccoloCAN's silly nullptr check

Require the library to supply the failure message (no default message)

Remove default cases so authors know to think about places they should
add things.

libraries/AP_KDECAN/AP_KDECAN.cpp

@@ -642,12 +642,12 @@ bool AP_KDECAN::pre_arm_check(char* reason, uint8_t reason_len)
 {
     if (!_enum_sem.take(1)) {
         debug_can(2, "KDECAN: failed to get enumeration semaphore on read\n\r");
-        snprintf(reason, reason_len ,"KDECAN enumeration state unknown");
+        snprintf(reason, reason_len ,"Enumeration state unknown");
         return false;
     }
 
     if (_enumeration_state != ENUMERATION_STOPPED) {
-        snprintf(reason, reason_len ,"KDECAN enumeration running");
+        snprintf(reason, reason_len ,"Enumeration running");
         _enum_sem.give();
         return false;
     }
@@ -665,17 +665,17 @@ bool AP_KDECAN::pre_arm_check(char* reason, uint8_t reason_len)
     uint8_t num_present_escs = __builtin_popcount(_esc_present_bitmask);
 
     if (num_present_escs < num_expected_motors) {
-        snprintf(reason, reason_len ,"Not enough KDECAN ESCs detected");
+        snprintf(reason, reason_len, "Too few ESCs detected");
         return false;
     }
 
     if (num_present_escs > num_expected_motors) {
-        snprintf(reason, reason_len ,"Too many KDECAN ESCs detected");
+        snprintf(reason, reason_len, "Too many ESCs detected");
         return false;
     }
 
     if (_esc_max_node_id != num_expected_motors) {
-        snprintf(reason, reason_len ,"Wrong KDECAN node IDs, run enumeration");
+        snprintf(reason, reason_len, "Wrong node IDs, run enumeration");
         return false;
     }