Rephrase error messages for consistency

test/asm/assert-nosect-bank.err

@@ -1,3 +1,3 @@
 error: assert-nosect-bank.asm(1):
-    PC has no bank outside a section
+    PC has no bank outside of a section
 error: Assembly aborted (1 error)!

test/asm/assert@-no-sect.err

@@ -1,3 +1,3 @@
 error: assert@-no-sect.asm(1):
-    PC has no value outside a section
+    PC has no value outside of a section
 error: Assembly aborted (1 error)!

test/asm/bracketed-symbols.err

@@ -3,5 +3,5 @@ error: bracketed-symbols.asm(16):
 error: bracketed-symbols.asm(20):
     "Label" does not have a constant value
 error: bracketed-symbols.asm(21):
-    Expected constant PC but section is not fixed
+    PC does not have a constant value; the current section is not fixed
 error: Assembly aborted (3 errors)!

test/asm/const-and.err

@@ -1,5 +1,5 @@
 error: const-and.asm(2):
-    PC has no value outside a section
+    PC has no value outside of a section
 error: const-and.asm(11):
     Expected constant expression: 'Aligned' is not constant at assembly time
 error: const-and.asm(17):

test/asm/endc-outside-if.err

@@ -1,2 +1,2 @@
 FATAL: endc-outside-if.asm(1):
-    Found ENDC outside an IF construct
+    Found ENDC outside of an IF construct

test/asm/undefined-builtins.err

@@ -1,9 +1,9 @@
 error: undefined-builtins.asm(3):
-    PC has no value outside a section
+    PC has no value outside of a section
 error: undefined-builtins.asm(4):
     Interpolated symbol "@" does not exist
 error: undefined-builtins.asm(8):
-    _NARG does not make sense outside of a macro
+    _NARG has no value outside of a macro
 error: undefined-builtins.asm(9):
     Interpolated symbol "_NARG" does not exist
 error: Assembly aborted (4 errors)!