Improve some RGBGFX error messages (#1876)

* Improve some RGBGFX error messages

* Fix assertion failure on ambiguous transparent/opaque pixels

test/gfx/ambiguous.err

@@ -0,0 +1,2 @@
+error: Color #ff800080 is neither transparent (alpha < 16) nor opaque (alpha >= 240) (first seen at (0, 8))
+Conversion aborted after 1 error

test/gfx/bad_slice.err

@@ -1,3 +1,3 @@
 error: Image slice ((2, 2) to (130, 130)) is outside the image bounds (20x20)
-note: Did you mean the slice "2,2:2,2"? (width and height are in tiles, not pixels!)
+       (Did you mean the slice "2,2:2,2"? The width and height are in tiles, not pixels!)
 Conversion aborted after 1 error

test/gfx/bg_fuse.err

@@ -1,3 +1,3 @@
-warning: Fusing colors #a9b9c9ff and #aabbccff into Game Boy color $66f5 [first seen at x: 1, y: 1]
+warning: Colors #a9b9c9ff and #aabbccff both reduce to the same Game Boy color $66f5 (first seen at (1, 1))
 FATAL: Tile (0, 0) contains the background color (#aabbccff)
 Conversion aborted after 1 error

test/gfx/gray_alpha.err

@@ -0,0 +1,2 @@
+FATAL: Image is not compatible with a DMG palette specification: it contains transparent pixels
+Conversion aborted after 1 error

test/gfx/gray_alpha.flags

@@ -0,0 +1 @@
+-c dmg

test/gfx/gray_conflict.err

@@ -0,0 +1,2 @@
+FATAL: Image is not compatible with a DMG palette specification: it contains a color #111111ff that reduces to the same gray shade as another one
+Conversion aborted after 1 error

test/gfx/gray_conflict.flags

@@ -0,0 +1 @@
+-c dmg

test/gfx/gray_nongray.err

@@ -0,0 +1,2 @@
+FATAL: Image is not compatible with a DMG palette specification: it contains a non-gray color #50555fff
+Conversion aborted after 1 error

test/gfx/gray_nongray.flags

@@ -0,0 +1 @@
+-c dmg

test/gfx/gray_too_many.err

@@ -0,0 +1,2 @@
+FATAL: Image is not compatible with a DMG palette specification: it contains too many colors (5)
+Conversion aborted after 1 error

test/gfx/gray_too_many.flags

@@ -0,0 +1 @@
+-c dmg