chg: better error message and error catching upon YAML merging

bin/compose

@@ -812,6 +812,7 @@ get_docker_compose_links() {
     merge_yaml_str "${deps[@]}" | tee "$cache_file" || return 1
     if [ "${PIPESTATUS[0]}" != 0 ]; then
         rm "$cache_file"
+        err "Failed to merge YAML from all ${WHITE}links${NORMAL} dependencies."
         return 1
     fi
 }
@@ -890,6 +891,7 @@ _get_docker_compose_service_mixin() {
         merge_yaml_str "$base_mixin" "$links_yaml" "$docker_compose_options" || return 1
     fi | tee "$cache_file"
     if [ "${PIPESTATUS[0]}" != 0 ]; then
+        err "Failed to constitute the base YAML for service '${DARKYELLOW}$service${NORMAL}'"
         rm "$cache_file"
         return 1
     fi
@@ -949,7 +951,10 @@ get_docker_compose () {
         debug "    ..finished all mixins for $DARKYELLOW$target_service$NORMAL $GRAY(in $((SECONDS - start))s)$NORMAL"
     done
 
-    docker_compose_services=$(merge_yaml_str "${entries[@]}") || return 1
+    docker_compose_services=$(merge_yaml_str "${entries[@]}") || {
+        err "Failed to merge YAML services entries together."
+        return 1
+    }
 
     base_v2="version: '2.0'
 networks:
@@ -984,7 +989,10 @@ _get_compose_service_def_cached () {
     value=$(echo "$docker_compose" | shyaml get-value "$service" 2>/dev/null)
     if ! echo "$value" | shyaml get-value "charm" >/dev/null 2>&1; then
         if charm.exists "$service"; then
-            value=$(merge_yaml <(echo "charm: $service") <(echo "$value")) || return 1
+            value=$(merge_yaml <(echo "charm: $service") <(echo "$value")) || {
+                err "Can't merge YAML infered 'charm: $service' with base ${DARKYELLOW}$service${NORMAL} YAML definition."
+                return 1
+            }
         else
             err "No ${WHITE}charm${NORMAL} value for service $DARKYELLOW$service$NORMAL" \
                 "in compose, nor same name charm found."
@@ -2101,7 +2109,10 @@ _get_docker_compose_mixin_from_metadata_cached() {
         image_or_build_statement="build: $(charm.get_dir "$charm")/build"
     fi
     if [ "$image_or_build_statement" ]; then
-        mixin=$(merge_yaml_str "$mixin" "$image_or_build_statement")
+        mixin=$(merge_yaml_str "$mixin" "$image_or_build_statement") || {
+            err "Failed to merge yaml with image or build YAML statement."
+            return 1
+        }
     fi
     echo "$mixin" | tee "$cache_file"
 }