reorganize a bit of early library and init code (#23256)

Remove base.jl, since it no longer had a clear purpose. Its contents
are moved to various more specific files.

Move docs for Core from helpdb to basedocs.jl.

base/array.jl

@@ -2,6 +2,17 @@
 
 ## array.jl: Dense arrays
 
+"""
+    DimensionMismatch([msg])
+
+The objects called do not have matching dimensionality. Optional argument `msg` is a
+descriptive error string.
+"""
+mutable struct DimensionMismatch <: Exception
+    msg::AbstractString
+end
+DimensionMismatch() = DimensionMismatch("")
+
 ## Type aliases for convenience ##
 """
     AbstractVector{T}

base/associative.jl

@@ -2,6 +2,16 @@
 
 # generic operations on associative collections
 
+"""
+    KeyError(key)
+
+An indexing operation into an `Associative` (`Dict`) or `Set` like object tried to access or
+delete a non-existent element.
+"""
+mutable struct KeyError <: Exception
+    key
+end
+
 const secret_table_token = :__c782dbf1cf4d6a2e5e3865d7e95634f2e09b5902__
 
 haskey(d::Associative, k) = in(k,keys(d))

base/boot.jl

@@ -135,7 +135,7 @@ export
     ErrorException, BoundsError, DivideError, DomainError, Exception,
     InterruptException, InexactError, OutOfMemoryError, ReadOnlyMemoryError,
     OverflowError, StackOverflowError, SegmentationFault, UndefRefError, UndefVarError,
-    TypeError,
+    TypeError, ArgumentError, MethodError, AssertionError, LoadError, InitError,
     # AST representation
     Expr, GotoNode, LabelNode, LineNumberNode, QuoteNode,
     GlobalRef, NewvarNode, SSAValue, Slot, SlotNumber, TypedSlot,
@@ -244,6 +244,40 @@ struct OverflowError <: Exception
     msg
 end
 
+mutable struct ArgumentError <: Exception
+    msg::AbstractString
+end
+
+mutable struct MethodError <: Exception
+    f
+    args
+    world::UInt
+    MethodError(@nospecialize(f), @nospecialize(args), world::UInt) = new(f, args, world)
+end
+const typemax_UInt = ccall(:jl_typemax_uint, Any, (Any,), UInt)
+MethodError(@nospecialize(f), @nospecialize(args)) = MethodError(f, args, typemax_UInt)
+
+mutable struct AssertionError <: Exception
+    msg::AbstractString
+    AssertionError() = new("")
+    AssertionError(msg) = new(msg)
+end
+
+#Generic wrapping of arbitrary exceptions
+#Subtypes should put the exception in an 'error' field
+abstract type WrappedException <: Exception end
+
+mutable struct LoadError <: WrappedException
+    file::AbstractString
+    line::Int
+    error
+end
+
+mutable struct InitError <: WrappedException
+    mod::Symbol
+    error
+end
+
 abstract type DirectIndexString <: AbstractString end
 
 String(s::String) = s  # no constructor yet