Make some error messages more consistent (#1393)

* Update some error messages

* Make non-A destination operand syntactically invalid

test/asm/ff00+c-bad.err

@@ -1,5 +1,7 @@
 error: ff00+c-bad.asm(8):
-    Expected constant expression equal to $FF00 for "$ff00+c"
+    Base value must be equal to $FF00 for $FF00+C
 error: ff00+c-bad.asm(9):
-    Expected constant expression equal to $FF00 for "$ff00+c"
-error: Assembly aborted (2 errors)!
+    Expected constant expression: 'xyz' is not constant at assembly time
+error: ff00+c-bad.asm(9):
+    Base value must be equal to $FF00 for $FF00+C
+error: Assembly aborted (3 errors)!

test/asm/invalid-instructions.err

@@ -1,17 +1,17 @@
 error: invalid-instructions.asm(1):
     Address $10000 is not 16-bit
 error: invalid-instructions.asm(2):
-    LD [HL],[HL] not a valid instruction
+    LD [HL], [HL] is not a valid instruction
 error: invalid-instructions.asm(3):
-    Expected constant expression equal to $FF00 for "$ff00+c"
+    Base value must be equal to $FF00 for $FF00+C
 error: invalid-instructions.asm(4):
-    Destination operand must be A
+    syntax error, unexpected c, expecting hl
 error: invalid-instructions.asm(5):
-    Destination operand must be A
+    syntax error, unexpected bc, expecting hl
 error: invalid-instructions.asm(6):
-    Destination operand must be A
+    syntax error, unexpected number, expecting hl
 error: invalid-instructions.asm(7):
-    Immediate value must be 3-bit
+    Bit number must be between 0 and 7, not 8
 error: invalid-instructions.asm(8):
     Invalid address $40 for RST
 error: Assembly aborted (8 errors)!

test/asm/invalid-jr.err

@@ -1,3 +1,3 @@
 error: invalid-jr.asm(3):
-    jr target out of reach (expected -129 < -258 < 128)
+    jr target must be between -128 and 127 bytes away, not -258; use jp instead
 error: Assembly aborted (1 error)!