New linespec error messages

[Keith Seitz]

Hi,

I'm nearly finished cleaning up my linespec rewrite. I have an enormous 
amount of bugs that I've fixed while writing some tests for it.

One thing that has evolved out of this work, though, is the ability to 
provide "better" error messages for the user. For example:

(gdb-head) b main.c:
Function "" not defined in "main.c".

(gdb-linespec-rewrite) b main.c:
malformed linespec error: unexpected EOF

Similarly, for other possible error conditions, my rewrite will 
(currently) output error messages such as:

malfomred linespec error: unexpected TOKEN[, "TOKEN_STRING"]

where TOKEN is one of the token types ("keyword", "string", "number", 
"EOF", "COLON", etc) and TOKEN_STRING is a string representation of what 
the user wrote. Here's another example:

(gdb-head) break main.c:3 +1000
Junk at end of arguments.

(gdb-linespec-rewrite) b main.c:3 +1000
malformed linespec error: unexpected number, "+1000"

Does anyone have any input on the text of these error messages? Any 
changes that maintainers might like?

Keith