Format CGB colors in error/verbose messages as "GB:rr,gg,bb"

These should be more debuggable than packed "$xxxx" format.

test/gfx/alpha_embedded.err

@@ -1,4 +1,4 @@
-error: Failed to fit tile colors [$1527, $15cc, $1ab4] in specified palettes
+error: Failed to fit tile colors [GB:07,09,05; GB:12,14,05; GB:20,21,06] in specified palettes
 note: The following palette was specified:
-        [$1ab4, $15cc]
+      - [GB:20,21,06; GB:12,14,05]
 Conversion aborted after 1 error

test/gfx/at-file-ref.err

@@ -1,8 +1,8 @@
-error: Failed to fit tile colors [$6c8a, $7f55, $7fff] in specified palettes
+error: Failed to fit tile colors [GB:10,04,27; GB:21,26,31; GB:31,31,31] in specified palettes
 note: The following palettes were specified:
-        [$5f77, $213d, $41a6, $40ee]
-        [$3f65, $36b3, $262a, $50b0]
-        [$53c3, $3f65, $36b3]
-        [$5f77, $267c, $41a6]
-        [$267c, $213d, $40ee]
+      - [GB:23,27,23; GB:29,09,08; GB:06,13,16; GB:14,07,16]
+      - [GB:05,27,15; GB:19,21,13; GB:10,17,09; GB:16,05,20]
+      - [GB:03,30,20; GB:05,27,15; GB:19,21,13]
+      - [GB:23,27,23; GB:28,19,09; GB:06,13,16]
+      - [GB:28,19,09; GB:29,09,08; GB:14,07,16]
 Conversion aborted after 1 error

test/gfx/bad_manual_pals.err

@@ -1,5 +1,5 @@
-error: Failed to fit tile colors [$6c8a, $7f55, $7fff] in specified palettes
+error: Failed to fit tile colors [GB:10,04,27; GB:21,26,31; GB:31,31,31] in specified palettes
 note: The following palettes were specified:
-        [$7fff, $7f55]
-        [$7fff, $6c8a]
+      - [GB:31,31,31; GB:21,26,31]
+      - [GB:31,31,31; GB:10,04,27]
 Conversion aborted after 1 error

test/gfx/bg_fuse.err

@@ -1,3 +1,3 @@
-warning: Colors #a9b9c9ff and #aabbccff both reduce to the same Game Boy color $66f5 (first seen at (1, 1))
+warning: Colors #a9b9c9ff and #aabbccff both reduce to the same RGB555 color GB:21,23,25 (first seen at (1, 1))
 FATAL: Tile (0, 0) contains the background color (#aabbccff)
 Conversion aborted after 1 error